Eighteen patients developed Staphylococcus aureus infection during or shortly after a five-month course of therapy with isotretinoin. Staphylococcus aureus was recovered both from lesions and from the anterior nares. In a prospective study, 4% of control patients treated with isotretinoin alone developed S aureus infection, while none who applied topical antibiotic to the anterior nares developed infection. Control patients had a 64% prevalence of anterior nares colonization with S aureus, compared with 18% in the topical antibiotic group.