The aim of this study was to investigate the extent to which the diagnosis of breast cancer influences the self-assessment of personality traits by patients. In 1989 and 1990, all female inhabitants of Nijmegen aged 43 years and over were invited to participate in a population screening for breast cancer and were asked to complete a personality questionnaire. During 1989-1992, 25 women who were diagnosed as having breast cancer were psychologically assessed for a second time 1 1/2 years after diagnosis. A control group of 822 healthy women also completed the questionnaire twice. A statistically significant decrease in scores on three scales (Rationality, Emotional Expression-out, and Emotional-Control) was reported by the patient group compared to the control group.