We investigate the association between psychological stress and breast cancer and, as oestrogen may provide a common mechanism, the association between stress and prostate cancer. A prospective study of 991 women and 5743 men employed in Scotland in the 1970s provided data. Risk exposure was measured by questionnaire and physical examination, routine data collection provided cancer outcomes over the subsequent 30 years. There was weak evidence of elevated incidences in those reporting moderate (breast cancer: hazard ratio [HR] 2.16, 95% CI 1.00-4.71; prostate cancer: HR 1.65, 95% CI 1.20-2.27) and high stress (breast cancer: HR 1.92, 95% CI 0.81-4.55; prostate cancer: HR 1.35, 95% CI 0.87-2.10) compared to those reporting low stress. These estimates are adjusted for socioeconomic circumstances and health-related behaviours. With no dose-response relationship and no established mechanism linking stress with breast and prostate cancer, confounding is the parsimonious explanation of these findings.