Of 335 women who underwent lumpectomy and radiation therapy for breast cancer, 42 subsequently developed calcifications. Particles typical of calcified suture material were identified in 21 of the 42 women (50%). No obvious calcified suture material was found in approximately 1,140 women of 38,000 (3%) who had undergone mammography after they had previously undergone breast biopsy for a benign lesion and thus had not undergone radiation therapy. Calcified suture material rarely develops in the nonirradiated breast, but it is common after radiation therapy and should not be confused with recurrent breast cancer. These calcifications are likely the result of delayed resorption of catgut sutures, which provide a matrix on which calcium can precipitate in a suitable local environment.