Recent neuropsychological studies have suggested that patients with early-treated phenylketonuria (PKU) and normal intelligence have a specific deficiency in solving complex spatial problems. In the present study a task involving the assembly of various shapes was used to compare the performance of 16 PKU patients and 11 sibling controls. Error rates generally were higher and response times slower among the PKU patients, but greater complexity did not produce differential changes in accuracy or speed in the PKU group compared to the controls. Correlations between task performance and IQ measures were significant for the PKU patients, but when IQ was controlled for the group differences vanished. The results suggest that choice of problem-solving strategy, attention span and accuracy of mental representation may be affected in PKU patients, despite efforts to maintain well-controlled phenylalanine concentrations in the blood.