Bilateral breast cancer (bBC) is the most common 2nd tumor in primary BC patients. However, its natural history is poorly understood as is the effect of previous adjuvant therapies. Between 1980 and 2005, we identified 3757 BC patients treated in our Institution, with 120 (3.2%) cases of bBC, 91 (2.4%) were metachronous BC (mBC), 29 (0.8%) synchronous BC (sBC). sBC defined as found before 3 months of the initial diagnosis. We performed a descriptive and an overall survival (OS) analysis. mBC appeared in young patients with a strong family history of BC. Most were diagnosed mammographically. The risk did not disappear after 15 years of follow-up. In most estrogen receptor (ER)-positive cases, the 2nd tumor was also ER-positive (concordance rate of 91%). No differences were seen according to the previous use of tamoxifen. In ER-negative cases, 43% of mBC were ER-positive. Synchronous BC (sBC) appeared in an elderly population with a strong family history. About 76% were ER-positive. ER status concordance was seen in 62%. There were no statistically significant differences in OS between patients with sBC or those with the 2nd mBC. A shorter time to appearance of the 2nd tumor predicted a worse OS. ER negativity and grade 3 tumors were negative prognostic factors. The risk of mBC does not abate with the pass of time. Contralateral mammographies should form part of follow-up. ER status concordance is high, especially in ER-positive cases. No differences were seen according to previous use of tamoxifen. Shorter disease-free intervals were linked with worse OS.