Alopecia is an important long-term complication after bone marrow transplantation (BMT). The aim of this study was to analyze the influence of busulfan concentration on the development of permanent alopecia. Sixty five patients who survived for at least 6 months after BMT were studied. The median follow-up was 2.1 years (range 0.5-5.7 years). Thirty one patients (47%) had some degree of alopecia and 19 of these patients had extensive alopecia. The mean minimum busulfan concentration was 656 +/- 222 ng/ml in patients who developed alopecia compared with 507 +/- 224 ng/ml in those who did not (P = 0.005). Patients with more extensive alopecia had higher busulfan concentrations than patients with less significant abnormalities. In multivariate analysis, alopecia was associated with busulfan concentrations higher than the median (OR 3.43; 95% CI 3.04-3.88), allogeneic transplantation (OR 2.56; 95% CI 2.28-2.88) and female sex (OR 1.96; 95% CI 1.73-2.88). There was no association between alopecia and chronic graft-versus-host disease. High busulfan concentrations may contribute to the development of permanent alopecia and the risk for alopecia should be considered when choosing the conditioning regimen before BMT.