Phenylketonuria (PKU) is a disorder characterized by several biochemical mechanisms which may impair the brain functions in PKU, leading to neurological problems. Our case report concerns a 19 year-old man with phenylketonuria who was evaluated with the onset of stiffness following the abandonment of the phenylalanine-restricted diet. He was assessed with grade-4 spasticity according to Modified Aschworth scale. The deep tendon reflexes had increased and the plantar reflexes were positive. Knee extensions were limited due to the shortening of the hamstring muscles. Serum phenylalanine concentration was elevated and plasma vitamin B(12) level was low. Cranial magnetic resonance imaging scan revealed demyelinization area in periventricular deep white matter. We administered a phenylalanine-restricted diet and a rehabilitation program. Following the treatment, spasticity was reduced to grade-1 and patient could walk without aid. This case shows that, the combination of diet, medication and a rehabilitation program is an effective treatment model on adult PKU with upper motor neuron involvement.