There is a general agreement that fibroadenoma is associated with a certain increase in breast cancer risk, but the risk estimates appear heterogeneous according to various studies. To provide further quantitative and population-based estimates of breast cancer risk in women with histologically confirmed fibroadenomas, we linked data of 1,461 incident cases of fibroadenoma (median age, 37 years) to the Cancer Registry of the Swiss Canton of Vaud (about 600,000 inhabitants) over the period 1977-1991. After exclusion of 57 synchronous breast neoplasms, a total of 17 cases of breast cancer were observed vs. 10.9 expected (standardised incidence ratio, SIR, 1.6; 90% confidence interval: 1.1-2.1). The SIR was similar in the short term (3-5 years) and in the medium-long term (> 5 years) following diagnosis of fibroadenoma. The cumulative risk of invasive breast cancer was 0.7% after 5 years and 2.2% after 12 years following diagnosis of fibroadenoma. This is not negligible on account of the young age of our cohort.