The bronchodilator effect of a 10 mg/kg dose of theobromine (3,7-dimethylxanthine) was compared with that of 5 mg/kg of theophylline (1,3-dimethylxanthine) in young patients with asthma. Bronchodilation, as assessed by forced vital capacity, forced expiratory volume in the first second, forced expiratory flows at 25%, 50%, and 75% of vital capacity, and percent of forced expiratory volume in the first second/forced vital capacity did not differ significantly between the two drugs. After each drug bronchodilation peaked at 2 hours and lasted for 6 hours, although it was not always statistically significant for theobromine. The mean peak serum concentrations of both drugs, the time at which peak serum concentrations occurred, and elimination half-life values were similar for theobromine and theophylline.