diff --git a/compiler+runtime/include/cpp/jank/runtime/obj/lazy_sequence.hpp b/compiler+runtime/include/cpp/jank/runtime/obj/lazy_sequence.hpp
index 919651d9b..17281f59c 100644
--- a/compiler+runtime/include/cpp/jank/runtime/obj/lazy_sequence.hpp
+++ b/compiler+runtime/include/cpp/jank/runtime/obj/lazy_sequence.hpp
@@ -1,5 +1,9 @@
 #pragma once
 
+#include <folly/Synchronized.h>
+
+#include <mutex>
+
 #include <jtl/option.hpp>
 
 #include <jank/runtime/object.hpp>
@@ -19,7 +23,8 @@ namespace jank::runtime::obj
 
     lazy_sequence() = default;
     lazy_sequence(lazy_sequence &&) noexcept = default;
-    lazy_sequence(lazy_sequence const &) = default;
+    lazy_sequence(lazy_sequence const &);
+    lazy_sequence &operator=(lazy_sequence const &);
     lazy_sequence(object_ref fn);
     lazy_sequence(object_ref fn, object_ref sequence);
 
@@ -46,6 +51,16 @@ namespace jank::runtime::obj
     lazy_sequence_ref with_meta(object_ref m) const;
 
   private:
+    struct state
+    {
+      object_ref fn{};
+      object_ref sv{};
+      object_ref s{};
+    };
+
+    folly::Synchronized<state, std::recursive_mutex>::LockedPtr lock_state() const;
+    void force_locked(folly::Synchronized<state, std::recursive_mutex>::LockedPtr &lock) const;
+
     object_ref resolve_fn() const;
     object_ref resolve_seq() const;
 
@@ -56,11 +71,8 @@ namespace jank::runtime::obj
     object_ref unwrap(object_ref ls) const;
 
   public:
-    /* TODO: Synchronize. */
     object base{ obj_type };
-    mutable object_ref fn{};
-    mutable object_ref sv{};
-    mutable object_ref s{};
+    mutable folly::Synchronized<state, std::recursive_mutex> state_;
     jtl::option<object_ref> meta;
   };
 }
diff --git a/compiler+runtime/src/cpp/jank/runtime/obj/lazy_sequence.cpp b/compiler+runtime/src/cpp/jank/runtime/obj/lazy_sequence.cpp
index 328597091..c900a4b17 100644
--- a/compiler+runtime/src/cpp/jank/runtime/obj/lazy_sequence.cpp
+++ b/compiler+runtime/src/cpp/jank/runtime/obj/lazy_sequence.cpp
@@ -11,54 +11,99 @@
 
 namespace jank::runtime::obj
 {
+  lazy_sequence::lazy_sequence(lazy_sequence const &other)
+  {
+    base = other.base;
+    meta = other.meta;
+    auto other_lock(other.lock_state());
+    auto lock(lock_state());
+    *lock = *other_lock;
+  }
+
+  lazy_sequence &lazy_sequence::operator=(lazy_sequence const &other)
+  {
+    if(this == &other)
+    {
+      return *this;
+    }
+
+    base = other.base;
+    meta = other.meta;
+    auto other_lock(other.lock_state());
+    auto lock(lock_state());
+    *lock = *other_lock;
+    return *this;
+  }
+
   lazy_sequence::lazy_sequence(object_ref const fn)
-    : fn{ fn }
   {
     jank_debug_assert(fn.is_some());
+    auto lock(lock_state());
+    lock->fn = fn;
   }
 
   lazy_sequence::lazy_sequence(object_ref const fn, object_ref const sequence)
-    : fn{ fn }
-    , s{ sequence }
   {
+    auto lock(lock_state());
+    lock->fn = fn;
+    lock->s = sequence;
+  }
+
+  folly::Synchronized<lazy_sequence::state, std::recursive_mutex>::LockedPtr
+  lazy_sequence::lock_state() const
+  {
+    return const_cast<folly::Synchronized<state, std::recursive_mutex> &>(state_).lock();
+  }
+
+  void lazy_sequence::force_locked(
+    folly::Synchronized<state, std::recursive_mutex>::LockedPtr &lock) const
+  {
+    if(lock->fn.is_nil())
+    {
+      return;
+    }
+
+    lock->sv = dynamic_call(lock->fn);
+    lock->fn = jank_nil;
   }
 
   object_ref lazy_sequence::seq() const
   {
     realize();
-    return s;
+    auto lock(lock_state());
+    return lock->s;
   }
 
   lazy_sequence_ref lazy_sequence::fresh_seq() const
   {
-    realize();
-    if(s.is_nil())
+    auto const sequence(seq());
+    if(sequence.is_nil())
     {
       return {};
     }
-    auto const r(runtime::fresh_seq(s));
+    auto const r(runtime::fresh_seq(sequence));
     jank_debug_assert(r != jank_nil);
     return make_box<lazy_sequence>(jank_nil, r);
   }
 
   object_ref lazy_sequence::first() const
   {
-    realize();
-    if(s.is_nil())
+    auto const sequence(seq());
+    if(sequence.is_nil())
     {
-      return s;
+      return sequence;
     }
-    return runtime::first(s);
+    return runtime::first(sequence);
   }
 
   object_ref lazy_sequence::next() const
   {
-    realize();
-    if(s.is_nil())
+    auto const sequence(seq());
+    if(sequence.is_nil())
     {
       return {};
     }
-    auto const n(runtime::next(s));
+    auto const n(runtime::next(sequence));
     return n;
   }
 
@@ -99,46 +144,43 @@ namespace jank::runtime::obj
 
   void lazy_sequence::realize() const
   {
-    /* TODO: Lock. */
-    force();
-    if(sv.is_some())
+    auto lock(lock_state());
+    force_locked(lock);
+    if(lock->sv.is_nil())
     {
-      auto ls{ sv };
-      sv = jank_nil;
-      if(ls.is_some() && ls->type == object_type::lazy_sequence)
-      {
-        ls = unwrap(ls);
-      }
-      s = runtime::seq(ls);
+      return;
     }
+
+    auto ls(lock->sv);
+    lock->sv = jank_nil;
+    if(ls.is_some() && ls->type == object_type::lazy_sequence)
+    {
+      ls = unwrap(ls);
+    }
+    lock->s = runtime::seq(ls);
   }
 
   void lazy_sequence::force() const
   {
-    if(fn.is_some())
-    {
-      sv = dynamic_call(fn);
-      fn = jank_nil;
-    }
+    auto lock(lock_state());
+    force_locked(lock);
   }
 
   void lazy_sequence::lock_and_force() const
   {
-    /* TODO: Lock */
-    force();
+    auto lock(lock_state());
+    force_locked(lock);
   }
 
   object_ref lazy_sequence::sval() const
   {
-    if(fn.is_some())
-    {
-      lock_and_force();
-    }
-    if(sv.is_some())
+    auto lock(lock_state());
+    force_locked(lock);
+    if(lock->sv.is_some())
     {
-      return sv;
+      return lock->sv;
     }
-    return s;
+    return lock->s;
   }
 
   object_ref lazy_sequence::unwrap(object_ref ls) const