To examine the effectiveness of erythromycin therapy and prophylaxis for pertussis, 17 households with one secondary case or more were compared with 20 households without secondary cases following a community-wide pertussis outbreak in Maricopa County, Arizona, in 1988. There were no significant differences between the two household groups in age distribution of members, size, crowding, race, proportion of children aged 7 months to 18 years with three or more diphtheria and tetanus toxoids and pertussis vaccine doses, or in the age distribution, vaccination status, or medical care of patients with primary cases. However, median intervals from onset of illness in primary cases to initiation of erythromycin therapy (for cases) and prophylaxis (for contacts) were 11 and 16 days, respectively, in households without secondary spread, vs 21 and 22 days, respectively, in households with secondary spread. These results provide additional evidence that erythromycin is effective in the medical management of pertussis and should be initiated promptly to minimize secondary spread.