Extramedullary relapses in children with acute lymphoblastic leukemia occur most frequently in the central nervous system and in the testis. In this report, the authors describe a 16-year-old girl with an isolated renal relapse of acute lymphoblastic leukemia after a disease-free interval of 2 years and 8 months. This clinically inconspicuous renal relapse was suggested by a routine follow-up renal sonography. No evidence of disease was found in bone marrow or peripheral blood. Renal biopsy was required to establish the diagnosis. Treatment consisted of intensive chemotherapy and autologous bone marrow transplantation. The patient has been in second complete continuous remission for 7 years. The authors recommend the use of an intensive multidrug salvage regimen.