diff --git a/eth2/utils/ssz/src/cached_tree_hash/resize.rs b/eth2/utils/ssz/src/cached_tree_hash/resize.rs
index d41453e9a5..a7bad0b04a 100644
--- a/eth2/utils/ssz/src/cached_tree_hash/resize.rs
+++ b/eth2/utils/ssz/src/cached_tree_hash/resize.rs
@@ -1,33 +1,112 @@
 use super::*;
 
 /// New vec is bigger than old vec.
-fn grow_merkle_cache(old_bytes: &[u8], old_flags: &[bool], to: usize) -> Option<Vec<u8>> {
-    let mut bytes = Vec::with_capacity(to * HASHSIZE);
-    let mut flags = Vec::with_capacity(to);
+fn grow_merkle_cache(
+    old_bytes: &[u8],
+    old_flags: &[bool],
+    from_height: usize,
+    to_height: usize,
+) -> Option<Vec<u8>> {
+    let to_nodes = (1 << to_height.next_power_of_two()) - 1;
 
-    let from = old_bytes.len() / HASHSIZE;
-    let to = to;
+    // Determine the size of our new tree. It is not just a simple `1 << to_height` as there can be
+    // an arbitrary number of bytes in `old_bytes` leaves.
+    let new_byte_count = {
+        let additional_from_nodes = old_bytes.len() / HASHSIZE - ((1 << from_height) - 1);
+        ((1 << to_height + additional_from_nodes) - 1) * HASHSIZE
+    };
+    dbg!(new_byte_count / 32);
 
-    let distance = (from.leading_zeros() - to.leading_zeros()) as usize;
+    let mut bytes = vec![0; new_byte_count];
+    let mut flags = vec![true; to_nodes];
 
-    let leading_zero_chunks = 1 >> distance;
+    let leaf_level = from_height - 1;
 
-    bytes.resize(leading_zero_chunks * HASHSIZE, 0);
-    flags.resize(leading_zero_chunks, true); // all new chunks are modified by default.
+    // Loop through all internal levels of the tree (skipping the final, leaves level).
+    for i in 0..from_height - 1 as usize {
+        dbg!(i);
+        dbg!(bytes.len());
+        // If we're on the leaf slice, grab the first byte and all the of the bytes after that.
+        // This is required because we can have an arbitrary number of bytes at the leaf level
+        // (e.g., the case where there are subtrees as leaves).
+        //
+        // If we're not on a leaf level, the number of nodes is fixed and known.
+        let old_slice = if i == leaf_level {
+            old_bytes.get(first_byte_at_height(i)..)
+        } else {
+            old_bytes.get(byte_range_at_height(i))
+        }?;
+
+        dbg!(byte_range_at_height(i + to_height - from_height));
+
+        let new_slice = bytes
+            .get_mut(byte_range_at_height(i + to_height - from_height))?
+            .get_mut(0..old_slice.len())?;
 
-    for i in 0..to.leading_zeros() as usize {
-        let new_slice = bytes.get_mut(1 >> i + distance..1 >> i + distance + 1)?;
-        let old_slice = old_bytes.get(1 >> i..1 >> i + 1)?;
         new_slice.copy_from_slice(old_slice);
     }
 
     Some(bytes)
 }
 
+fn byte_range_at_height(h: usize) -> Range<usize> {
+    first_byte_at_height(h)..last_node_at_height(h) * HASHSIZE
+}
+
+fn first_byte_at_height(h: usize) -> usize {
+    first_node_at_height(h) * HASHSIZE
+}
+
+fn first_node_at_height(h: usize) -> usize {
+    (1 << h) - 1
+}
+
+fn last_node_at_height(h: usize) -> usize {
+    (1 << (h + 1)) - 1
+}
+
 #[cfg(test)]
 mod test {
+    use super::*;
+
     #[test]
     fn can_grow() {
-        // TODO
+        let from: usize = 7;
+        let to: usize = 15;
+
+        let old_bytes = vec![42; from * HASHSIZE];
+        let old_flags = vec![false; from];
+
+        let new = grow_merkle_cache(
+            &old_bytes,
+            &old_flags,
+            (from + 1).trailing_zeros() as usize,
+            (to + 1).trailing_zeros() as usize,
+        )
+        .unwrap();
+
+        println!("{:?}", new);
+        let mut expected = vec![];
+        // First level
+        expected.append(&mut vec![0; 32]);
+        // Second level
+        expected.append(&mut vec![42; 32]);
+        expected.append(&mut vec![0; 32]);
+        // Third level
+        expected.append(&mut vec![42; 32]);
+        expected.append(&mut vec![42; 32]);
+        expected.append(&mut vec![0; 32]);
+        expected.append(&mut vec![0; 32]);
+        // Fourth level
+        expected.append(&mut vec![0; 32]);
+        expected.append(&mut vec![0; 32]);
+        expected.append(&mut vec![0; 32]);
+        expected.append(&mut vec![0; 32]);
+        expected.append(&mut vec![0; 32]);
+        expected.append(&mut vec![0; 32]);
+        expected.append(&mut vec![0; 32]);
+        expected.append(&mut vec![0; 32]);
+
+        assert_eq!(expected, new);
     }
 }