The effect of early erythromycin treatment was studied in 63 whooping cough patients. They were all culture‐positive in the catarrhal stage and/or two weeks or less after the onset of coughing. 28 patients were treated with eryhtromycin for 7–10 days: children with a dose of 40 mg/kg/day and adults with 1g/day. 35 patients received no antibiotics. 152 Bordetella pertussis cultures were obtained from the untreated patients and 70 from the patients treated with eryhtromycin.The erythromycin‐treated patients were culture‐negative on average 1‐2 weeks earlier than the untreated patients. Of the patients treated with erythromycin, 6/28 (21%) were still culture‐positive after treatment. This study shows that erythromycin treatment may reduce the period during which patients are infectious and that the treatment should be started two weeks from the onset of coughing, or prefera