diff --git a/msgspec/_core.c b/msgspec/_core.c
index 5e9804c7..022b2e1c 100644
--- a/msgspec/_core.c
+++ b/msgspec/_core.c
@@ -5140,17 +5140,23 @@ rename_camel_inner(PyObject *field, bool cap_first) {
     bool first = true;
     for (Py_ssize_t i = 0; i < PyList_GET_SIZE(parts); i++) {
         PyObject *part = PyList_GET_ITEM(parts, i);
-        if (PyUnicode_GET_LENGTH(part) == 0) continue;
-        if (!first || cap_first) {
-            /* convert part to title case, inplace in the list */
-            PyObject *part_title = PyObject_CallMethod(part, "title", NULL);
-            if (part_title == NULL) goto cleanup;
-            PyList_SET_ITEM(parts, i, part_title);
+        if (first && (PyUnicode_GET_LENGTH(part) == 0)) {
+            /* Preserve leading underscores */
+            PyList_SET_ITEM(parts, i, underscore);
             Py_DECREF(part);
         }
-        first = false;
+        else {
+            if (!first || cap_first) {
+                /* convert part to title case, inplace in the list */
+                PyObject *part_title = PyObject_CallMethod(part, "title", NULL);
+                if (part_title == NULL) goto cleanup;
+                PyList_SET_ITEM(parts, i, part_title);
+                Py_DECREF(part);
+            }
+            first = false;
+        }
     }
-    empty = PyUnicode_FromString("");
+    empty = PyUnicode_FromStringAndSize("", 0);
     if (empty == NULL) goto cleanup;
     out = PyUnicode_Join(empty, parts);
 
diff --git a/tests/test_struct.py b/tests/test_struct.py
index f3c64076..300ec428 100644
--- a/tests/test_struct.py
+++ b/tests/test_struct.py
@@ -1942,12 +1942,14 @@ class Test(Struct, rename="kebab"):
             field_two_with_suffix: str
             __field_three__: bool
             field4: float
+            _field_five: int
 
         assert Test.__struct_encode_fields__ == (
             "field-one",
             "field-two-with-suffix",
             "field-three",
             "field4",
+            "field-five",
         )
 
     def test_rename_camel(self):
@@ -1956,12 +1958,14 @@ class Test(Struct, rename="camel"):
             field_two_with_suffix: str
             __field__three__: bool
             field4: float
+            _field_five: int
 
         assert Test.__struct_encode_fields__ == (
             "fieldOne",
             "fieldTwoWithSuffix",
-            "fieldThree",
+            "__fieldThree",
             "field4",
+            "_fieldFive",
         )
 
     def test_rename_pascal(self):
@@ -1970,12 +1974,14 @@ class Test(Struct, rename="pascal"):
             field_two_with_suffix: str
             __field__three__: bool
             field4: float
+            _field_five: int
 
         assert Test.__struct_encode_fields__ == (
             "FieldOne",
             "FieldTwoWithSuffix",
-            "FieldThree",
+            "__FieldThree",
             "Field4",
+            "_FieldFive",
         )
 
     def test_rename_callable(self):