This is a double-blind, randomized controlled trial at an urban pediatric emergency department. Sample size (162) was based on a threshold equivalence of 7% (α = 0.05, power = 80%). Inclusion criteria were: non-toxic, immunocompetent, 3 months to 18 years old, English-speaking patients with clinical or ultrasound identified skin abscesses who were not on antibiotics. Patients were block randomized to receive placebo or trimethoprim/sulfamethoxazole following incision and drainage. Follow-up was a call at 2-3 days & a repeat visit or call at 10-14 days. Treatment failure was defined as: persistent erythema, tenderness, and/or draining lesions. New lesion was defined as: primary resolution with development of new lesion (furuncle, carbuncle or abscess) at a different location. Compliance was evaluated by the return of the study medication or by patient report.