SummaryWe describe agoraphobia as a complication of phenylketonuria (PKU) in young adults. The five patients have classic PKU and received phenylalanine‐restricted diet only in childhood. Only one has normal intelligence. All but one were also depressed. All were anxious. Three of the five had initiated the phenylalanine‐restricted diet after 3 months of age. Two returned to the phenylalanine‐restricted diet with dramatic reduction of symptoms.The frequency of manifestations of agoraphobia was also examined in 50 young women with PKU enrolled in a longitudinal study of psychosocial factors in maternal PKU, 47 of their acquaintances and 49 women with diabetes. All were administered a test of agoraphobic‐avoidant behaviour. The women with PKU appeared to be more prone to social withdrawal and fear of leaving home. Twenty per cent were within the agoraphobia range of the Mobility Inventory. Those still on diet and those with non‐PKU hyperphenylalaninaemia reported less avoidant behaviour than those who had terminated the diet in childhood.These results suggest that young adults with PKU are at risk for agoraphobia but that return to the phenylalanine‐restricted diet may be an effective treatment.