Occupational factors have been suggested as potential causes of breast cancer. We examined the risk of breast cancer by occupation and in relation to occupational exposure to petroleum and chlorinated solvents, which have been implicated as potential risk factors. We used data from two population-based case-control studies, one of which focused on breast cancer in men (104 cases and 1,901 controls across 8 European countries), and the other on breast cancer in women (1,230 cases and 1,315 controls in Côte-d'Or and Ille-et-Vilaine). A complete career history was available in each study. Exposures to petroleum and chlorinated solvents were assessed using an occupation-exposure matrix developed by the Department of Occupational Health (DST) at the InVS. Odds ratios adjusted for known risk factors for breast cancer were calculated. In the analyses by occupation, the risk of breast cancer among men was elevated among motor vehicle mechanics, suggesting a possible carcinogenic role for petroleum products. An increased risk of breast cancer among women was also suggested among nurses, textile workers, and rubber and plastics workers, but the increases were not statistically significant. The incidence of breast cancer was lower among female farmers. Furthermore, occupational exposure to benzene and trichloroethylene was associated with an increased risk of breast cancer in men, but these associations were not confirmed in women, who had much lower occupational exposure levels. Overall, these results support the hypothesis that exposure to organic solvents at relatively high levels may play a role in the development of breast cancer. (R.A.)