Since more and more children survive allogeneic bone marrow transplantation (BMT), knowledge of acute and late complications becomes increasingly important. Besides the major complications [(opportunistic) infections, veno-occlusive disease, graft versus host disease, and recurrence of primary disease], acute and chronic renal insufficiency are significant post-transplant complications that may contribute to transplant-related mortality. To elucidate risk factors for acute and chronic renal insufficiency post BMT, we performed a prospective study of all 66 children who received a BMT in a 2-year period at our institution; 21% had acute renal insufficiency post BMT. Risk factors for acute renal insufficiency were veno-occlusive disease, high cyclosporin serum levels, and foscarnet therapy. Of surviving patients, 11% developed chronic renal insufficiency 1 year post BMT. Acute renal insufficiency was the sole predictor of chronic renal insufficiency. In contrast to studies in adults, we did not find total body irradiation to be a risk factor for chronic renal insufficiency. Future long-term studies are needed to assess incidence and morbidity of chronic renal insufficiency in children following allogeneic BMT.