The study aims were to determine the use of complementary therapies (CT) by men with prostate cancer, and to explore factors influencing CT use and attitudes toward CT use. A cross-sectional survey design was used in which a postal questionnaire was mailed to an eligible sample of 405 patients with prostate cancer receiving outpatient treatment in a London teaching hospital. The primary outcomes were the prevalence of CT use and the relationship between CT use and mental health status. Two hundred and ninety-four patients (73%) responded, of whom 25% were using CT. The most frequently used CTs were vitamins, low-fat diets, lycopene and green tea. Multivariate analyses revealed no differences in mental health scores between CT users and non-users. CT users were younger (OR 0.93, 95% CI 0.89-0.97) and were more likely to be receiving conservative management in the form of 'active surveillance' (OR 5.23, 95% CI 1.78-15.41) compared with non-users. Over half of the participants (55%) wanted to learn more about CT. Forty-three per cent of CT users had not informed any doctor about their CT use. Clinicians need to be aware of the prevalence of CT use amongst patients with prostate cancer, considering the potential harm that could be caused by interactions with conventional treatments.