Renal insufficiency is a common complication early after hematopoietic stem cell transplantation ( HSCT). Renal function as measured by creatinine clearance (CrCl) was prospectively evaluated in 47 patients undergoing allogeneic (n = 22) or autologous (n = 25) HSCT during the first 100 days. Renal dysfunction was classified as follows: Grade 0 (< 25% decline in CrCl), Grade 1 (>= 25% decline in CrCl but < 2 x increase in serum creatinine), Grade 2 (>= 2 x rise in serum creatinine but no need for dialysis) and Grade 3 (>= 2 x rise in serum creatinine and need for dialysis). Thirty-three patients (70%) had Grade 1-3 renal dysfunction. Renal dysfunction was more common after myeloablative allogeneic HSCT (91%) than autologous HSCT (52%) (P = 0.004), and was associated with a high risk of mortality (P = 0.039). Sepsis in autologous HSCT patients and cyclosporine toxicity in allogeneic HSCT patients were associated with renal dysfunction. We conclude that autologous and allogeneic HSCT differ in the likelihood and causes of renal dysfunction.