The aim of this study was to determine the prevalence, severity and correlates of fatigue in a convenience sample of outpatients with prostate cancer prior to and following 3-months treatment with first-line hormone therapy (cyproterone acetate and goserelin). 'Severe fatigue' in the patients (n=62) was defined as a score on the Fatigue Severity Scale (FSS) greater than the 95th percentile of a group of elderly volunteers without cancer. Subjects also completed other questionnaires about fatigue and about quality of life, anxiety/depression and personality. Subjects underwent a nutritional assessment, tests of voluntary muscle function and attention. The prevalence of 'severe fatigue' at baseline was 8/58 (14%). Median FSS scores increased significantly after 3 months treatment. On multivariate analysis psychological distress explained 28% of the variance in fatigue scores. Treatment was associated with a reduction in voluntary muscle function, loss of muscle bulk, a decline in virility and potency, an improvement in pain and a reduction in nausea/vomiting. Fatigue is an important but under-recognised side-effect of hormone therapy.