Biochemical bone markers are a valuable noninvasive tool useful in the management of metabolic bone diseases. Phenylketonuria (PKU) is a disorder in which dietary phenylalanine restriction is the only known therapy for the prevention of mental impairment in patients. This kind of diet may influence the bone metabolism, especially in childhood and adolescence, when growth and bone turnover are most intensive. Some authors have described in patients with phenylketonuria reduced mineralization and other abnormalities of bone tissue. We measured osteocalcin and bone alkaline phosphatase (formation markers) and collagen type I crosslinked C-telopeptide (resorption marker) in 33 prepubertal children with PKU and in 65 prepubertal healthy children. Significantly lower serum osteocalcin concentration (83.9 +/- 32.9 ng/mL) in PKU patients than in control group (103.2 +/- 17.0 ng/mL) (p<0.01) was observed. Activity of serum bone alkaline phosphatase was similar in both studied groups. Serum concentration of collagen type I crosslinked C-telopeptide was significantly lower in PKU patients than in healthy children and amounted to 12790 +/- 4332 pmol/L and 16190 +/- 2815 pmol/L, respectively (p<0.001). Further studies of the bone mineral density and metabolism in prepubertal children with PKU are needed in order to prevent osteopenia and osteoporosis in adult patients.