The incidence of delayed breast abscess as a complication following the treatment of breast cancer has not been reported. A retrospective review of 112 patients (pts) undergoing lumpectomy and radiation therapy (RT) in our institution revealed a six per cent incidence of delayed breast abscess (range 1.5-8 months, median 5 months). Prophylactic antibiotics (P = 1.0), postoperative chemotherapy (P = 1.0), primary vs. re-excisional lumpectomy (P = 1.0), and different surgeons (P = 0.514) were not associated with increased risk of delayed abscess. All abscesses occurred in the first 32 pts of this series. The size of the lumpectomy cavity correlated with the incidence of infection (P = 0.0440). Since six of seven abscess cultures grew staphylococci (coagulase negative three pts, coagulase positive three pts), and four of these pts experienced prior biopsy site infection, skin necrosis or repeated seroma aspirations, a skin source for contamination was suggested. Treatment of the abscesses with antibiotics and immediate drainage produced acceptable but inferior cosmesis. We conclude that a small but significant subset of patients treated with lumpectomy and RT will develop delayed wound infections and that expeditious treatment affords satisfactory cosmesis.