Eighteen patients, who had previously taken part in a 2 wk cross-over comparison between formoterol and salbutamol, now took part in a one year double-blind study comparing salbutamol 200 micrograms b.i.d. with formoterol 12 micrograms b.i.d. Additional doses were taken when needed and were recorded. Ten patients were started on formoterol and eight on salbutamol. After one month, the patients were allowed to shift to the alternative drug. Two patients withdrew from the study. At the end of the study, 13 of 16 patients were on formoterol, thus showing a long-lasting preference for this drug. Forced expiratory volume in one second (FEV1) dose-response curves for inhaled salbutamol were repeatedly recorded during the study, and no tachyphylaxis was found. One patient stopped taking inhaled steroids but continued taking formoterol and theophylline. He deteriorated with a decreased response to salbutamol. After re-introduction of inhaled steroids his condition improved. This case indicates that effective bronchodilator therapy may mask the deterioration of asthma.