Fenoterol ( Berotec ; Th 1165a , Boehringer- Ingelheim , Spa., Firenze , Italy) is an effective, widely used bronchodilator. It has been suggested by some investigators that, as in the case of other beta-adrenergic agonists, its clinical usefulness may be limited by the development of tolerance; others have found no tolerance. These conflicting results appear to be related to differences in study designs. We studied for 3 months the effects of inhaled fenoterol in 15 asthmatics, using a rigidly controlled protocol. Appropriate serial physiologic measurements were made at regular intervals during the 90-day study. In all instances the day 1 bronchodilator response was significant, prompt, and sustained: at 1,3,4,8 and 12 wk the response was statistically the same as on day 1. It is concluded that, when the important variables are properly controlled, no evidence of tolerance to long-term therapy with fenoterol is demonstrable.