Abstract Purpose Tamoxifen reduces the risk of contralateral breast cancer (CBC), but little is known about the long-term risk. Methods From the database under the Danish Breast Cancer Cooperative Group (DBCG), we identified 15,863 users and 21,670 nonusers of tamoxifen among women diagnosed with breast cancer during 1977 to 2007. Information on CBC was obtained from the Danish Cancer Registry and the Danish Breast Cancer Cooperative Group database. Cox regression analyses were carried out with estimation of hazard ratios (HRs) of CBC. We also estimated the HRs of current and past use of tamoxifen. Results Current use of tamoxifen was associated with reduced HRs of CBC independently of menopausal status and calendar-period. The HRs associated with past use of tamoxifen, however, varied among postmenopausal users from 1.50 (95% confidence interval CI, 1.20–1.87) among users in 1977 to 1989 to 0.87 (95% CI, 0.70–1.06) and 1.05 (95% CI, 0.68–1.63) among users in 1990 to 1998 and 1999 to 2007, respectively. No excesses or deficits of CBC were found 10 or more years since the last use in postmenopausal women in any calendar-period. Conclusions Our study confirms that tamoxifen protects against CBC while being treated. The effect of tamoxifen after cessation of treatment was not clear because of the inconsistency in results by calendar-period.