This study investigates the histopathological characteristics of a consecutive series of 100 screen detected breast cancers in relation to residual disease. Tumour type, size, grade, resection margins and extent of primary or associated in situ disease were all assessed by one pathologist. Thirty-seven women underwent further surgery (wider excision or mastectomy) and the resected specimen was examined for residual in situ or invasive cancer. In total, 36 cancers had an extensive in situ component, of which 69% were predominantly intraduct carcinoma of comedo type. Of the 37 women who underwent further resection, 21 (57%) women had residual cancer. Of those with initial disease at the resection margin, 16/25 (64%) had residual disease. Five of 12 (42%) with disease close to (within 2-3 mm) but not at the margin had residual disease. Of those with an extensive in situ component, 18/25 (72%) had residual disease, whereas only 2/12 (17%) women with none or some in situ disease had residual cancer. In screen detected breast cancer, residual cancer was present in 72% of women with an extensive in situ component at initial surgery. These women comprise a group in which conservation surgery may be inappropriate if completeness of excision is considered a prerequisite for breast conserving surgery.