Various personality characteristics have been suggested to increase the risk of breast cancer but reliable epidemiologic data on this issue are limited. We prospectively investigated the relationship between personality characteristics and the risk of breast cancer in 12,499 Finnish women aged 18 years or more. In health questionnaires in 1975 and 1981, these women completed at least one of the following personality scales: Eysenck extroversion, Bortner type A behaviour and author-constructed measure of hostility. They also reported about other potential breast cancer risk factors. From 1976-1996, 253 cases of breast cancer were identified by record linkage with the Finnish Cancer Registry. Proportional hazard models were used to estimate hazard ratios (HR) and 95% confidence intervals (CI). The multivariable HRs of breast cancer for women with intermediate level (scores 3-6) and high level (7-9) of extroversion in 1975 were 1.18 (95% CI 0.87-1.60) and 0.97 (95% CI 0.64-1.47), respectively, compared to those with low level (0-2). These results remained unaltered when the level of extroversion was determined as the average of the 1975 and 1981 reports. There was also no increase in breast cancer risk in relation to type A behaviour and hostility. Furthermore, we observed no substantial joint effects of personality characteristics on the risk of breast cancer. In conclusion, our data do not support the existence of an important role for personality in the aetiology of breast cancer. These findings are reassuring to those who have believed the contrary.