To investigate the usefulness of computed tomography (CT) with skin-marker placement in determining the excision area and decreasing the positive or close margin rates in breast-conserving surgery (BCS). Multidetector-row helical computed tomography (MDCT) mapping images were reconstructed in subjects (n = 117) diagnosed with primary breast cancer who had undergone MDCT using CT skin markers. Serial 5-mm-thick slices prepared from the surgical specimen were used for pathological analyses. A "positive margin" was defined as the presence of malignant cells at the surgical margin, and a "close margin" as a tumor within 5 mm of the surgical margin. The rates of positive and close margins were calculated. We identified the lesions in 111 of 117 cases (94.9%) on MDCT. Of these, 93 underwent BCS under the guidance of MDCT mapping and the remaining 18 underwent mastectomy. Among the 93 cases, 6 (6.5%) had positive or close margins and were diagnosed with ductal carcinoma in situ of low nuclear grade. MDCT mapping with a CT skin marker is feasible for simulating surgical positioning and determining the excision area. MDCT mapping could decrease the positive and close margin rates in BCS.