A 41-year-old woman had meaningful functional improvement following reinstitution of a low phenylalanine diet. She was diagnosed at birth with phenylketonuria and followed strict dietary adherence till the age of 16. Thereafter the diet was discontinued. She subsequently presented with subacute profound visual loss, cognitive dysfunction and paraparesis such that she was bed bound requiring full nursing care. Following dietary intervention there was meaningful improvement such that she was no longer demented and while her vision remains poor she is now independent for activities of daily living. This case report suggests that consideration of reimplementation of dietary intervention is warranted even after a prolonged period of time.