To evaluate the subacute effects of heavy marihuana smoking on the lung, pulmonary function was tested in 28 healthy young male experienced cannabis users, before and after 47 to 59 days of daily ad-libitum marihuana smoking (mean of 5.2 marihuana cigarettes per day per subject, 2.2 per cent delta9-tetrahydrocannabinol). Base-line pulmonary-function studies were nearly all within normal limits, but after 47 to 59 days of heavy smoking, statistically significant decreases in forced expired volume in one second (3 +/- 1 per cent, S.E.), maximal mid-expiratory flow rate (11 +/- 2 per cent), plethysmographic specific airway conductance (16 +/- 2 per cent) and diffusing capacity (8 +/- 2 per cent) were noted as compared with the base-line studies. The decreases in maximal mid-expiratory flow rate and specific airway conductance were correlated with the quantity of marihuana smoked. These findings suggest that customary social use of marihuana may not result in detectable functional respiratory impairment in healthy young men, whereas very heavy marihuana smoking for six to eight weeks causes mild but statistically significant airway obstruction.