Bone turnover markers and bone mineral density (BMD) were studied in 25 adult patients (14 females, 11 males) who had undergone allogeneic bone marrow transplantation (BMT). The interval from BMT to the first examination was at least 1 year (mean 3, range 1-10). Mean age of the patients at the time of first evaluation was 42 (range 19-54) years. Blood samples and urine collections for evaluation of biochemical factors reflecting skeletal turnover were performed together with the first BMD measurement. BMD was measured from the lumbar vertebrae (L2 to L4) with computed tomography and results were expressed as Z-scores. At the time of the first measurement five patients (20%) had Z-scores <-2.5 s.d. and 12 patients (48%) between -1 and -2.5 s.d. In 12 patients BMD assessments were repeated and it seemed that reduction in BMD had mostly occurred during and shortly after BMT and remained the same during follow-up. The cross-linked carboxyterminal telopeptide of type I collagen (ICTP) correlated negatively with BMD (r = -0.45, P = 0.045) as did bone-specific alkaline phosphatase (BAP; r = -0.64, P = 0.002). No correlation between BMD and time interval from diagnosis to BMT, conditioning regimen, corticosteroid use or hospital stay during transplantation was found. In conclusion, bone disease is common after BMT. Our findings demonstrate an increased collagen and bone turnover and a high risk of osteoporosis. BMD measurements must be repeated regularly and collagen markers such as ICTP and BAP can be beneficial in estimating the activity of bone disease.