This report describes the follow-up of 108 women who underwent ipsilateral mastectomy for lobular carcinoma in situ (LCIS). Twenty-four women found to have concurrent contralateral carcinoma underwent bilateral mastectomy. The contralateral breast was available for follow-up in the remaining 84 cases, including 33 patients who underwent contralateral biopsy and 51 others who did not have a biopsy of the opposite breast at the time of initial treatment. Five of these 84 patients later developed invasive cancer. Three had had a biopsy that revealed either atypia (two cases) or LCIS (one case). In the two other cases, there had not been a prior biopsy. Two of the 26 patients who had had a benign breast biopsy were found to have LCIS, but none subsequently had intraductal or invasive carcinoma, and none of the 26 women died of breast carcinoma. In this series, 64% of the women retained their contralateral breast; deaths due to contralateral breast carcinoma occurred with half the frequency that had been observed in a prior study of women with LCIS who did not have a contralateral biopsy. These results tend to support our current recommendation to treat LCIS by ipsilateral mastectomy and contralateral biopsy. However, it would be necessary to study these patients for approximately ten more years before results can be considered conclusive. Concurrently, prospective controlled investigations should be pursued to confirm these results, to identify patients most at risk in developing invasive carcinoma, and to determine whether nonsurgical therapy can modify the course of LCIS.