In this double-blind, randomised trial conducted in 22 centres in the USA, azithromycin given over five days, as a once-a-day regimen, (500 mg on day 1, 250 mg on days 2-5) was compared with cephalexin (500 mg b.i.d.) given for ten days in the treatment of patients with skin and skin structure infections. A total of 366 patients entered the study and 179 of these were eligible for the efficacy analysis. The overall clinical response to azithromycin was 94.0%, compared with 95.8% for cephalexin. The clinical cure rates were 53.0% for azithromycin and 59.4% for cephalexin; the respective improvement rates were 41.0% and 36.5%. Distribution of response (cured, improved, failed) was similar in each group (p = 0.37). The bacteriological eradication rate for azithromycin-treated patients was 94.2% and for cephalexin-treated patients was 90.3% (p = 0.34). Clinical and bacteriological response was similar in each group for all primary diagnoses. The two antibiotics were well tolerated, the overall incidence of side effects being 13.7% with approximately 60% due to gastrointestinal disturbances. In all but one case (cephalexin) the severity of the reported side effects was mild or moderate. Six patients withdrew from the study due to treatment-related events; five had been treated with azithromycin and one with cephalexin. In summary, a five-day, once-daily regimen of azithromycin was as effective as a ten-day, twice-daily regimen of cephalexin in the treatment of patients with skin and skin structure infections.