Second breast cancer experience was examined for 27,175 primary breast cancer patients diagnosed in the State of Connecticut during 1935-75 with follow-up for second breast cancers through 1980. The overall ratio of observed to expected second breast cancers was 3.2. Relative risk was found to be inversely related to age at diagnosis and directly related to stage of the first breast cancer. The overall risk of second breast cancers was 711 per 100,000 person-years at risk. Risk of second breast cancers showed a similar relationship to age and stage as relative risk and was also directly related to calendar period of diagnosis of the first breast cancer. Some interactions were observed because patients less than 45 years old at diagnosis with positive nodes had elevated risks and relative risks in the early followup period, whereas less of an effect of stage on relative risk and risk was seen for older patients. The effect of the use of adjunctive radiation therapy on second breast cancer risk was also assessed by the ratio of the risk of second breast cancers for those patients who received both surgery and radiation to the risk of those patients who only received surgery being estimated for patients diagnosed during 1935-59 and for patients diagnosed during 1960-75. For both cohorts relative risks of 1.2-1.4 were found for the 5-year period immediately following diagnosis, likely resulting from the uncontrolled effect of stage in the analysis. Elevated long-term relative risks were not found for patients diagnosed during 1935-59. A long-term marginally statistically significant relative risk of 1.4 (greater than or equal to 10 yr after diagnosis) was found for patients diagnosed during 1960-75. The data do not indicate an overall pattern of relative risks consistent with an effect on long-term second breast cancer risk of radiation exposure to the opposite breast incurred during adjunctive radiation therapy for a first breast cancer.