An adult male with intellectual disabilities demonstrated deterioration in many skills over a number of years and an increase in his temper outbursts was also reported. At 18 years of age, he had been diagnosed as having dihydropteridine reductase (DHPR) deficiency, but treatment had proved unsuccessful in the short term and was discontinued. Dihydropteridine reductase deficiency is a recessively inherited disorder of the amino acid metabolism resulting in a deficiency of tetrahydrobiopterin, an essential cofactor for phenylalanine, tyrosine and tryptophan metabolism. This causes a severe deficiency of neurotransmitters in the brain. Following further neurological examinations, treatment for the subject was recommenced at the age of 30 years. Few reports of late-diagnosis DHPR have been documented. This paper outlines one case report of DHPR, highlighting the importance of diagnosis, medical treatment and nursing care.