The study was performed on 13 asthmatic patients to determine whether inhaled dipyridamole would act directly by inducing bronchoconstriction or indirectly by potentiating the adenosine-induced bronchoconstriction. The study was performed in 3 consecutive days. On the first day adenosine challenge was performed and the PD20 value calculated. On the other days the adenosine challenge was done 5 min after randomized inhalations of dipyridamole or a control solution. The mean percent change in FEV1 after dipyridamole (delta % = 2.0) and control solution (delta % = 1.0) was not significant. Inhaled adenosine caused bronchoconstriction with a geometric mean PD20 of 1.09 mg. After control solution inhalation, a mean PD20 value of 1.31 mg was observed. Dipyridamole inhalation increased adenosine hyperresponsiveness and in all subjects shifted the dose-response curves of adenosine challenge to the left with a mean PD20 value of 0.40 mg. This enhancing effect of dipyridamole was significant when compared with the baseline value (P less than 0.01) and control solution (P less than 0.01). The study demonstrated that dipyridamole inhalation increased airway responsiveness to adenosine in all subjects. This effect is due to indirect activity of dipyridamole on airways without changes in baseline airway caliber.