The aim of this randomized, double-blind, parallel group study was to determine the lowest effective dose of 6, 12 and 24 microg formoterol fumarate dihydrate Turbuhaler b.i.d. compared with placebo. The 4 week treatment was preceded by a 1 week run-in period. Morning peak expiratory flow (PEF) before intake of the study drug was the primary variable. Patients recorded PEF, prior to and 15 min after intake of the study drug (immediate response), asthma symptoms, and use of rescue medication morning and evening. Of 221 patients (71 females and 150 males), 194 were included in the efficacy per protocol (PP) analysis; mean age 47 yrs, mean forced expiratory volume in one second (FEV1) 2.01 L (58% of predicted), mean FEV1 reversibility 27% at entry. Ninety percent used inhaled steroids. Compared with placebo, 6 microg formoterol b.i.d was found to be the lowest effective dose in the morning (p=0.008) and evening p=0.0041) PEF. The mean increases in PEF were 22 and 23 L x min-1 respectively, compared with placebo. After 6 microg formoterol, the mean immediate increase in morning PEF was 42 L x min-1 compared to an increase of only 9 L x min-1 after placebo (<0.0001). All doses produced a statistically significant decrease in asthma symptoms, day and night, and the need for rescue medication at night. All doses were well-tolerated. In conclusion, the lowest effective dose in this study was formoterol Turbuhaler 6 microg b.i.d.