A 7-year-old boy with asthma was receiving the leukotriene receptor antagonist pranlukast (Ultair; SmithKline Beecham; Pittsburgh) as part of an open-label clinical trial. The patient's asthma improved, and he remained asymptomatic; but routine study evaluations 9 to 12 months into therapy showed microhematuria, proteinuria, glucosuria, anemia, and renal insufficiency. Renal biopsy demonstrated changes classic for acute allergic tubulointerstitial nephritis (ATIN), with mixed interstitial inflammatory infiltrate including eosinophils. Within 6 months of pranlukast withdrawal, anemia resolved and urinary sediment and renal function normalized. The case demonstrates that hypersensitivity reaction to pranlukast and resultant ATIN is possible, and that periodic urine testing in patients receiving pranlukast should be considered.