diff --git a/envstruct.go b/envstruct.go
--- a/envstruct.go
+++ b/envstruct.go
@@ -14,7 +14,6 @@
 
 	if prefix != "" {
 		envVarBuf.WriteString(prefix)
-		envVarBuf.WriteRune('_')
 	}
 
 	skip := false
@@ -39,23 +38,25 @@
 // so that your environment variables do not clash with others.
 //
 // Automatically converts struct field names to environment variable case, using
-// the following algorithm: the prefix is added, if any, then non-consecutive
-// uppercase letters in the field name are preceded by an underscore. Example:
-// the value for field UseJSON is read from USE_JSON. Nested structs are handled
-// similarly: the field name is added on as the struct tree is traversed.
-// Example: if UseJSON is inside a struct in field Log, the value is read from
-// LOG_USE_JSON.
+// the following algorithm: non-consecutive uppercase letters in the field name
+// are preceded by an underscore. Example: the value for field UseJSON is read
+// from USE_JSON. Nested structs are handled similarly: the field name is added
+// on as the struct tree is traversed. Example: if UseJSON is inside a struct in
+// field Log, the value is read from LOG_USE_JSON.
 //
 // The fields of s may have the struct tags envVar and envDefault.
 //
 //   - If a field has the envVar struct tag, the value for that field is read
-//     from the environment variable named envVar. prefix is not prepended to
-//     envVar.
+//     from the environment variable named envVar.
 //
 //   - If a field has the envDefault struct tag, and the environment variable
 //     is not defined, the field is initialized with the value of envDefault.
 //     If the environment variable is not defined and envDefault is not set,
 //     returns an error.
+//
+// If prefix is not empty, both autogenerated and custom environment variable
+// names are prefixed unless the field has the tag envNoPrefix:"true". prefix is
+// converted to uppercase, but is otherwise not modified.
 //
 // All fields in s must be string, uint, bool, or implement
 // [encoding.TextUnmarshaler]. Panics otherwise. Returns an error if an
@@ -63,6 +64,7 @@
 //
 // Will only handle fields exported from the struct.
 func Parse(prefix string, s any) error {
+	prefix = strings.ToUpper(prefix)
 	v := reflect.ValueOf(s).Elem()
 	st := v.Type()
 	if st.Kind() != reflect.Struct {
@@ -75,9 +77,15 @@
 
 		envVar := f.Tag.Get("envVar")
 		envDefault := f.Tag.Get("envDefault")
+		p := prefix + "_"
+		if f.Tag.Get("envNoPrefix") == "true" {
+			p = ""
+		}
 
 		if envVar == "" {
-			envVar = convertCase(prefix, f.Name)
+			envVar = convertCase(p, f.Name)
+		} else {
+			envVar = p + envVar
 		}
 		vf := v.FieldByName(f.Name)
 		if f.Type.Kind() == reflect.Struct {
diff --git a/envstruct_test.go b/envstruct_test.go
--- a/envstruct_test.go
+++ b/envstruct_test.go
@@ -8,7 +8,7 @@
 )
 
 func TestConvertCase(t *testing.T) {
-	const prefix = "MY_APP"
+	const prefix = "MY_APP_"
 	var tests = []struct {
 		input    string
 		expected string
@@ -28,7 +28,7 @@
 		})
 		t.Run(fmt.Sprintf("%s with prefix", tt.input), func(t *testing.T) {
 			output := convertCase(prefix, tt.input)
-			expected := prefix + "_" + tt.expected
+			expected := prefix + tt.expected
 			if output != expected {
 				t.Errorf("got %s, expected %s", output, expected)
 			}
@@ -45,7 +45,7 @@
 		Port    uint
 		BufSize uint
 		Log     logOpts
-		Custom  string `envVar:"MY_VAR"`
+		Custom  string `envVar:"MY_VAR" envNoPrefix:"true"`
 		IP      net.IP
 	}
 	type inputs struct {
@@ -66,7 +66,6 @@
 		{
 			"simple",
 			inputs{"443", "64", "~/.cache/my_app/log", "0", "fun", "127.0.0.1"},
-			// testStruct{443, 64, "~/.cache/my_app/log", false, "fun", net.IPv4(127, 0, 0, 1)},
 			testStruct{443, 64, logOpts{"~/.cache/my_app/log", false}, "fun", net.IPv4(127, 0, 0, 1)},
 			false,
 		},
@@ -96,21 +95,20 @@
 		},
 	}
 
-	// TODO: update tests and docs
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			var s testStruct
 
-			t.Setenv("PORT", tt.values.port)
-			t.Setenv("BUF_SIZE", tt.values.bufsize)
+			t.Setenv("PREFIX_PORT", tt.values.port)
+			t.Setenv("PREFIX_BUF_SIZE", tt.values.bufsize)
 			if tt.values.logfile != "" {
-				t.Setenv("LOG_FILE", tt.values.logfile)
+				t.Setenv("PREFIX_LOG_FILE", tt.values.logfile)
 			}
-			t.Setenv("LOG_USE_JSON", tt.values.useJSON)
+			t.Setenv("PREFIX_LOG_USE_JSON", tt.values.useJSON)
 			t.Setenv("MY_VAR", tt.values.custom)
-			t.Setenv("IP", tt.values.ip)
+			t.Setenv("PREFIX_IP", tt.values.ip)
 
-			err := Parse("", &s)
+			err := Parse("prefix", &s)
 			t.Log(s)
 			if err != nil && !tt.errExpected {
 				t.Errorf("got parse error: %s", err)