Because the introduction of PSA testing has increased the reported incidence of prostate cancer, this study assessed the mental impact on men after receiving a diagnosis of prostate cancer. Participants in a prostate cancer screening trial (ERSPC) completed a questionnaire on health and, if diagnosed with prostate cancer, at two additional time points. In the pre-screening phase 3800 men (response 88%) completed the questionnaire. Of screen-diagnosed men (n=82) 52 (response 63%) completed two additional assessments. Gleason scores were < or=7 in 96% of men. Mental and self-rated overall health worsened significantly immediately after diagnosis (P < or=0.04). Six months later these scores improved and no longer differed significantly from the pre-diagnosis score. After obtaining a pre-diagnosis assessment in prospective prostate cancer patients we found a significant negative mental impact of prostate cancer diagnosis based on PSA testing. We recommend that clinicians share their knowledge on the generally favourable prognosis with their patients. The methodological implication is that considering a post-diagnosis assessment as 'baseline' may lead to an underestimation of the patient's mental health.