Local recurrence after lumpectomy for ductal carcinoma in situ (DCIS) is a major concern and is related to residual disease in the breast. We studied the predictive value of lumpectomy margins for residual DCIS and compared our results and pathological processing techniques with those published in the literature. Margin status was determined for 89 patients with screen-detected DCIS who had lumpectomy and re-excision, for the presence and extent of residual disease. Margin width was defined as the narrowest distance between tumor and any inked margin or, where margins were positive, classified into focal involvement (<1 mm of the inked surface involved), minimal (>or=1<15 mm) and extensive (>or=15 mm). The amount of residual tumor was quantified according to the number of ducts involved with tumor: small (fewer than 10 ducts) or large (10 or more ducts) residuum. The initial margin status was a significant predictor for the presence of residual tumor in re-excision specimens (P=0.006). There was residual tumor in 44 and 45% of close non-involved (>1 and <or=1 mm width) margins, 67% of focally, 71% of moderately and 94% of extensively positive margins. The pathologic tumor size was also a predictor for the presence of residual tumor with 27, 68 and 74% of lesions measuring <or=10, 11-25, >25 mm,respectively, showing residual disease. The presence of residual tumor was not significantly related to age, mammographic appearance, nuclear grade or intraductal necrosis. The initial margin status was found to predict for the amount of residual tumor. With careful margin assessment, margin status after lumpectomy for DCIS can be used to predict for the presence and amount of residual tumor in the breast and is a guide to further management decisions. A standard for margin status reporting and pathological processing of screen-detected DCIS in situ lesions will help in the interpretation of data from different institutions.