Background: When phenylketonuria (PKU) is not diagnosed and long-term treatment commenced within the first few weeks of life, permanent brain damage will occur. There is some evidence to show that late diagnosed or untreated people with PKU who have severe challenging behaviours may benefit from a low phenylalanine diet [Harper & Reid (1987) "Journal of Mental Deficiency Research" vol. 31, pp. 209-212; Hoskins et al. (1992) "Journal of Intellectual Disability Research" vol. 36, pp. 183-191; Baumeister & Baumeister (1998) "Clinical Neuropharmacology" vol. 21, pp. 18-27; Fitzgerald et al. (2000) "Journal of Intellectual Disability Research" vol. 44, pp. 53-59]. Method: In the light of this evidence, a woman with late-diagnosed PKU was treated with a low-phenylalanine diet in an attempt to reduce her severe challenging behaviour. Results: A reduction in challenging behaviours was observed when phenylalanine levels were maintained within a specific narrow range. Data are reported over a 2-year period. A dramatic rise in severe challenging behaviour was noticed when her blood phenylalanine levels fell below a certain level. Conclusions: This case report suggests that blood phenylalanine levels need to be maintained within a specific range for maximum benefit.