Five asymptomatic patients studied 2 to 12 days after recovering from pulmonary edema associated with heroin overdose had statistically significant reduction of vital capacity and total lung capacity to 49% and 56% of predicted, respectively. The diffusing capacity for carbon monoxide was decreased, but diffusion per unit of lung volume was increased. In two patients, studied 10 to 12 weeks later, these abnormalities persisted. The diffusing capacity was significantly reduced in long-term addicts without pulmonary edema, suggesting that these patients may suffer from functionally detectable, but clinically asymptomatic, chronic lung disease.