A 24-year-old female patient with rapidly progressive renal failure secondary to membrano-proliferative glomerulonephritis is thought to have subsequently had acute interstitial pneumonitis as a consequence of azathioprine therapy. The occurrence of the pulmonary injury coincided with the rapid onset of severe renal failure and was reversible with cessation of the drug. Accumulation of toxic metabolic products of azathioprine with subsequent injury to the pulmonary interstitium is suggested as the etiology. Azathioprine should be considered in the differential diagnosis of interstitial pneumonitis in all patients taking the drug.