Clenbuterol was compared to an aminophylline preparation in a double-blind crossover trial involving 47 patients with asthma and reversible airways obstruction. Following a 2-week control period, each drug was given for a 4-week period. The patients made daily records of the severity of symptoms and recorded PEFR morning and night. Both drugs produced a highly significant reduction in severity and duration of wheeze by day during the first 4 weeks, but only clenbuterol produced further significant reduction in these symptoms during the second 4 weeks. Only clenbuterol produced a significant reduction in the severity of wheeze at night. When clenbuterol was given first there was a significant increase in the mean PEFR both night and morning but no other significant changes with either drug at other periods. Combining the results from the two treatment groups showed a significant difference between them in favour of clenbuterol in respect of duration of daytime wheeze only (p less than 0.05).