Out of 6084 first breast cancers registered between 1974 and 1993 in the Cancer Registry of the Swiss Canton of Vaud, 81 (1.3%) were synchronous bilateral breast cancers (BBCs). The 5-year relative survival rates were 73% for women with unilateral breast cancer (UBC) and 65% for those with synchronous BBC. The corresponding 10-year figures were 59% and 51%. This large, population-based series indicates that women with synchronous BBC have a moderately lower long-term survival than women with UBC. [authors]