diff --git a/mongo/address/addr.go b/mongo/address/addr.go
index fb6abbcd7c..85cf12067c 100644
--- a/mongo/address/addr.go
+++ b/mongo/address/addr.go
@@ -29,8 +29,11 @@ func (a Address) Network() string {
 // String is the canonical version of this address, e.g. localhost:27017,
 // 1.2.3.4:27017, example.com:27017.
 func (a Address) String() string {
-	// TODO: unicode case folding?
-	s := strings.ToLower(string(a))
+	s := string(a)
+	if a.Network() != "unix" {
+		// TODO: unicode case folding?
+		s = strings.ToLower(string(a))
+	}
 	if len(s) == 0 {
 		return ""
 	}
diff --git a/mongo/address/addr_test.go b/mongo/address/addr_test.go
index 2242b6d49c..0e2f474ce1 100644
--- a/mongo/address/addr_test.go
+++ b/mongo/address/addr_test.go
@@ -22,6 +22,7 @@ func TestAddress_String(t *testing.T) {
 		{"A:27017", "a:27017"},
 		{"a:27017", "a:27017"},
 		{"a.sock", "a.sock"},
+		{"A.sock", "A.sock"},
 	}
 
 	for _, test := range tests {
diff --git a/testdata/connection-string/valid-unix_socket-absolute.json b/testdata/connection-string/valid-unix_socket-absolute.json
index 5bb02476eb..66491db13b 100644
--- a/testdata/connection-string/valid-unix_socket-absolute.json
+++ b/testdata/connection-string/valid-unix_socket-absolute.json
@@ -30,6 +30,21 @@
       "auth": null,
       "options": null
     },
+    {
+      "description": "Unix domain socket (mixed case)",
+      "uri": "mongodb://%2Ftmp%2FMongoDB-27017.sock",
+      "valid": true,
+      "warning": false,
+      "hosts": [
+        {
+          "type": "unix",
+          "host": "/tmp/MongoDB-27017.sock",
+          "port": null
+        }
+      ],
+      "auth": null,
+      "options": null
+    },
     {
       "description": "Unix domain socket (absolute path with spaces in path)",
       "uri": "mongodb://%2Ftmp%2F %2Fmongodb-27017.sock",
diff --git a/testdata/connection-string/valid-unix_socket-absolute.yml b/testdata/connection-string/valid-unix_socket-absolute.yml
index 651e06fe39..cdfd00d332 100644
--- a/testdata/connection-string/valid-unix_socket-absolute.yml
+++ b/testdata/connection-string/valid-unix_socket-absolute.yml
@@ -23,6 +23,18 @@ tests:
                 port: ~
         auth: ~
         options: ~
+    -
+        description: "Unix domain socket (mixed case)"
+        uri: "mongodb://%2Ftmp%2FMongoDB-27017.sock"
+        valid: true
+        warning: false
+        hosts:
+            -
+                type: "unix"
+                host: "/tmp/MongoDB-27017.sock"
+                port: ~
+        auth: ~
+        options: ~
     -
         description: "Unix domain socket (absolute path with spaces in path)"
         uri: "mongodb://%2Ftmp%2F %2Fmongodb-27017.sock"
diff --git a/testdata/connection-string/valid-unix_socket-relative.json b/testdata/connection-string/valid-unix_socket-relative.json
index 2ce649ffc2..788720920b 100644
--- a/testdata/connection-string/valid-unix_socket-relative.json
+++ b/testdata/connection-string/valid-unix_socket-relative.json
@@ -30,6 +30,21 @@
       "auth": null,
       "options": null
     },
+    {
+      "description": "Unix domain socket (mixed case)",
+      "uri": "mongodb://rel%2FMongoDB-27017.sock",
+      "valid": true,
+      "warning": false,
+      "hosts": [
+        {
+          "type": "unix",
+          "host": "rel/MongoDB-27017.sock",
+          "port": null
+        }
+      ],
+      "auth": null,
+      "options": null
+    },
     {
       "description": "Unix domain socket (relative path with spaces)",
       "uri": "mongodb://rel%2F %2Fmongodb-27017.sock",
diff --git a/testdata/connection-string/valid-unix_socket-relative.yml b/testdata/connection-string/valid-unix_socket-relative.yml
index 7164188b3b..6d0d108205 100644
--- a/testdata/connection-string/valid-unix_socket-relative.yml
+++ b/testdata/connection-string/valid-unix_socket-relative.yml
@@ -23,6 +23,18 @@ tests:
                 port: ~
         auth: ~
         options: ~
+    -
+        description: "Unix domain socket (mixed case)"
+        uri: "mongodb://rel%2FMongoDB-27017.sock"
+        valid: true
+        warning: false
+        hosts:
+            -
+                type: "unix"
+                host: "rel/MongoDB-27017.sock"
+                port: ~
+        auth: ~
+        options: ~
     -
         description: "Unix domain socket (relative path with spaces)"
         uri: "mongodb://rel%2F %2Fmongodb-27017.sock"