Previous studies have suggested that children with phenylketonuria (PKU) have a reduction in bone mineralization compared with control subjects. To investigate this, bone mineral density (BMD) of the total body (TBMD) was measured in 32 prepubertal children with PKU and in 95 age-matched control subjects. Spine bone mineral density (SBMD) was also recorded in a subset, 24 with PKU and 55 control subjects. The effect of dietary intake on bone mass was assessed in 30 of the children with PKU and in 12 control subjects. In the children with PKU, TBMD and SBMD were significantly lower than in the control subjects after adjustment for height and weight (P = 0.03 and P = 0.003, respectively). The children with PKU had a higher intake of calcium (P < 0.0001), phosphorus (P = -0.0002), and magnesium (P < 0.0001), suggesting that their lower BMD occurred despite an adequate diet based on current recommendations. Further study is needed to establish the cause of this deficit in bone mass and the benefit of additional nutritional support to reverse this problem.