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Bone mineral status was assessed in 48 children with phenylketonuria (PKU) (20M, 28 F, aged 2.5-17 y). Bone density was measured in the distal third of the right forearm using single photon absorptiometry and was expressed as +/-SD with respect to age- and gender-matched controls. Serum calcium (Ca), magnesium (Mg), phosphorus (P), alkaline phosphatase (ALP), parathyroid hormone and 25-hydroxyvitamin D were measured in morning samples. The ratios of urinary Ca/creatinine (UCa/UCr), UP/UCr, UMg/UCr and hydroxyproline (OH-Pr)/UCr were calculated in urine samples collected over a period of 3 h. Patients' data were compared with those of 50 controls (22 M, 28 F, aged 3-15 y). The data showed severe osteopenia (below -2SD) in 22/48 patients. Bone loss was more prominent in patients over 8 y old. Bone density correlated significantly with age (r=-0.56,p < 0.001) and with Phe (r=-0.49, p < 0.007) but did not correlate with the other biochemical indices studied. Comparing PKU children with controls, significantly higher serum calcium and magnesium (p=0.04, p < 0.001, respectively), lower ALP (p=0.01), higher UCa/UCr ratio (p < 0.001), lower UP/UCr (p < 0.001) and lower UOH-Pr/UCr (p < 0.001) were found. Dietary compliance was poor in patients over the age of 8y, as only 3/22 of < or = 8y had mean serum phenylalanine >10mgdl(-1), in contrast to 21/26 in the older group. It is clear from the data that osteopenia is commonly found in PKU patients from early life. The biochemical data indicate a metabolic state of low bone turnover in PKU patients. In conclusion, a better, more restricted diet may correct osteopenia. [ABSTRACT FROM AUTHOR]