We considered the risk of subsequent invasive breast cancer in a population-based series of 579 carcinomas in situ (CIS) of the breast (482 ductal, 88 lobular) registered between 1977 and 2002 in the Swiss Canton of Vaud. A total of 55 cases of invasive breast cancer were observed vs. 12.3 expected, corresponding to a standardized incidence ratio (SIR) of 4.5 (95% confidence interval [CI], 3.4-5.8). The SIR was 4.6 after ductal and 4.2 after lobular CIS, was similar with passing time since CIS diagnosis, but was higher (SIR = 5.5) for women aged <55 years. At 20 years following CIS, the cumulative risk of invasive breast cancer was 26%, similar for lobular and for ductal CIS. The incidence of invasive breast cancer following CIS showed no consistent pattern of trends with age, all rates in subsequent age groups ranging between 10 and 18 in 1,000. This is compatible with the occurrence of a single mutational event in a population of susceptible women. [Ed.]