After the Food and Drug Administration approved a topical antibiotic for the treatment of acne, we began a postmarketing surveillance study to measure the frequency of antibiotic-associated colitis and diarrhea in acne patients treated with topical antibiotics and low doses of oral antibiotics. Pharmacists recruited 13,465 patients who presented a prescription for an acne medication, and we obtained detailed information on the use of the medications, perceived acceptability, and the occurrence of new health events from 6,453 with computer-assisted telephone interviews. Three cases (less than 0.1%) of antibiotic-associated diarrhea and one case (less than 0.1%) of antibiotic-associated colitis were confirmed. We conclude that this methodology can provide further information about the safety of a drug once it is in customary use, at a fraction of the cost of Phase III clinical trials.