To investigate the effects of chronic marijuana smoking on lung function, pulmonary function tests including single-breath carbon monoxide diffusing capacities were performed in 15 healthy women who smoked 1.7 +/- 1.4 (mean +/- SD) marijuana cigarettes per day for 235 +/- 135 days per year for a mean of 10.5 +/- 3.7 years. Control groups included 27 nonsmoking and 26 tobacco-smoking women. Results revealed that marijuana smoking with or without tobacco is associated with a reduction in the single-breath carbon monoxide diffusing capacity to 74 +/- 20 percent of predicted, which was significantly different from that in the nonsmoking control subjects (92 +/- 11 percent; p less than 0.05). The subset of subjects who smoked marijuana and tobacco had a further reduction of the single-breath carbon monoxide diffusing capacity to 65 +/- 17 percent, which was significantly different from that in both nonsmoking and smoking control subjects (80 +/- 7 percent). These results suggest that heavy marijuana smoking when added to tobacco smoking may damage the gas exchange surface of the lung.