We performed a double-blind study on the effect of oral administration of 10 mg of delta9-tetrahydrocannabinol on specific airway conductance (Gaw/VL) and the maximal expiratory flow at 50% of vital capacity (Vmax 50%) in six control and six asthmatic subjects. In control subjects, there was a slight but statistically significant increase in Gaw/VL after oral administration of delta9-tetrahydrocannabinol; however, there was no significant increase in Vmax 50%. One of the asthmatic patients developed severe bronchoconstriction following administration of delta9-tetrahydrocannabinol; among the remaining five patients, there were variable changes in Gaw/VL and Vmax 50% after oral administration of delta9-tetrahydrocannabinol, but mean changes were not significant. Mild effects on the central nervous system (CNS) were observed in three subjects; six subjects, three of whom had unpleasant mood changes, had more prominent CNS effects. We concluded that oral administration of delta9-tetrahydrocannabinol is unlikely to be of therepeutic value in asthma, since its bronchodilator action was mild and inconstant and was associated with significant CNS effects. Moreover, one asthmatic patient developed severe bronchoconstriction following oral administration of delta 9-tetrahydrocannabinol.