One hundred and thirty seven patients presenting with genital ulcerations from which Haemophilus ducreyi was isolated were treated with erythromycin stearate 500 mg every six hours for seven days. Of these, 91 (66%) had associated inguinal lymphadenopathy. Only two of the 100 patients who returned after one week showed no clinical improvement. Despite decrease in size H ducreyi was reisolated from the ulcers of three patients, two of whom had not complied with treatment. The patients were treated for a further week either with erythromycin or with a placebo preparation and on day 14 no discernible difference in clinical response was evident. H ducreyi was not reisolated from any lesion. In contrast, the natural course of development of associated lymphadenopathy was not modified by treatment. H ducreyi was not, however, isolated from any gland after the start of treatment. Side effects attributable to erythromycin were minimal and treatment had to be discontinued in only two patients. This study clearly indicates that treatment with erythromycin for one week results in rapid healing of lesions and the elimination of H ducreyi from both ulcers and associated lymph glands.