It is still not completely clear whether or not carbamazepine (CBZ) causes alterations in vitamin D status and in bone metabolism. The objective of this study was therefore to investigate prospectively in healthy adults the effects of CBZ on serum levels of 25-hydroxyvitamin D (25OHD) and on biomarkers of bone formation and resorption. Twenty-one free-living male adults were taking 800 mg/day CBZ for 10 weeks. The study was performed from December 1997 until September 1998 at a geographic latitude of 51 degrees N. Blood samples were collected before treatment (t1), 33 days (SE 2.5) after starting treatment (t2), and 70 days (SE 3.6) after starting treatment (t3). In 13 out of the 21 subjects blood samples were also drawn 64 days (SE 9.0) after treatment had been terminated (t4). Serum 25OHD levels remained constant during study periods t1-t3. 25OHD levels were, however, significantly higher at t4 compared to t1-t3. Serum concentrations of intact osteocalcin, a bone formation marker, and C-telopeptide, a bone resorption marker, were similar during all examinations. Moreover, serum levels of parathyroid hormone, calcium, and inorganic phosphate did not change. Data indicate that CBZ per se does not alter bone metabolism and does not lead to decreased circulating 25OHD levels in young males without epilepsy. Variations in 25OHD levels are in line with the seasonal fluctuations in vitamin D status.