A 10-year-old boy with osteosarcoma and normal renal function manifested laboratory evidence of impending renal toxicity and extreme elevation of aspartate aminotrasferase and alanine aminotransferase within 2 hours after the completion of a 4-hour infusion of high-dose methotrexate (MTX) (12 g/m2), and went on to develop acute renal failure with life-threatening hyperkalemia 29 hours later. Although his renal function recovered completely with high-dose leucovorin, hemodialysis, charcoal hemoperfusion, and carboxypeptidase G2, we present this case to emphasize that signs of renal toxicity may be present as early as 2 hours after the completion of a 4-hour MTX infusion, and to suggest that monitoring for MTX toxicity should perhaps begin within a few hours after the completion of 4-hour MTX infusion.