We report a 5-year-old female who presented with unexplained acute renal failure (ARF) and hyperuricemia and who was subsequently diagnosed of T-cell acute lymphoblastic leukemia (ALL). Peripheral smear was initially unremarkable. She required hemodialysis. Two weeks later, peripheral smear showed 40% blasts and bone marrow demonstrated T-cell ALL. Our case was the fifth and the youngest case of ALL with spontaneous tumor lysis syndrome. However, in contrast to previous reports in ALL or acute myeloid leukemia, our patient did not have blasts noted on periphereal blood smear and her white blood cell count and serum lactate dehydrogenase level were normal on admission, a time when dialysis-dependent ARF and severe hyperuricemia were present. Occult hematologic malignancy should be considered in cases of ARF and hyperuricemia of unknown etiology even when peripheral hematologic findings are not informative.