We studied urate metabolism in 36 patients undergoing both allogeneic and autologous bone marrow transplantation (BMT) without allopurinol. Most patients had low tumour burdens. Three different preparative regimens were used; busulphan/cyclophosphamide (BUCY); BCNU, etoposide, ara-C and melphalan (BEAM) and cyclophosphamide/total body irradiation (CY/TBI). Urate excretion rose during each of the regimens but the pattern of excretion varied with each regimen. Urate excretion remained elevated 72 h after completion of BEAM and BUCY, possibly reflecting the prolonged action of some of the agents used, e.g. melphalan, busulphan and etoposide. Urinary urate concentrations were unchanged compared with pre-chemotherapy levels reflecting the adequacy of the hydration protocol. No significant rise in creatinine was seen and no cases of tumour lysis syndrome occurred. Serum uric acid levels were a poor reflection of urate production, falling in most patients, and are an unreliable end-point for decisions regarding prophylaxis. BMT can be safely undertaken in patients with low tumour loads without allopurinol if an adequate urine volume is maintained. In this series, high levels of urate excretion often persisted for 72 h after the completion of conditioning and adequate hydration should be ensured during this period.