The aim of the study was to investigate the time of onset and the duration of the bronchodilating effect of different doses of formoterol administered via Turbuhaler in patients with moderate asthma. Thirty-one patients (five women) with a mean forced expiratory volume in 1 s (FEV1) of 1.97 +/- 0.54 1 and a mean reversibility of 31 +/- 14% of baseline were included in this double-blind, randomized, placebo-controlled and cross-over study. The patients inhaled single doses of placebo, i.e. 6, 12, 24, or 48 micrograms formoterol fumarate, on 5 separate days. Serial measurements of specific airways conductance (SGAW) and FEV1 were performed at regular time intervals for 12 h. The majority of the patients had at least a 50% increase in SGAW within 1-4 min after administration of all active treatments. The maximum increase in FEV1 over placebo was dose-dependent: 12% (6 micrograms), 18% (12 micrograms), 19% (24 micrograms), and 26% (48 micrograms) (P < 0.001). Twelve hours after administration of 6, 12, 24, and 48 micrograms formoterol, the mean increase in FEV1 was still 7%, 15%, 18%, and 27%, respectively, above the value following placebo. Headache was the most frequently reported adverse event in all treatments including placebo. After inhalation of 48 micrograms, three patients experienced mild tremor lasting for less than 1 h; likewise, one patient experienced the same event for 3 h after placebo. Formoterol administered via Turbuhaler10 gave a rapid and dose-related bronchodilating effect lasting for 12 h and was well tolerated.