OBJECTIVE To systematically review controlled trials investigating the effects of exercise interventions in cancer patients. METHODS Studies were located through searching seven electronic databases (Medline, Embase, Cochrane Library, CancerLit, PsycInfo, Cinahl, SportDiscus), scanning reference lists of relevant articles, contacting experts (n = 20), and checking the contents lists of journals available through ZETOC (Electronic Table of Contents). To be included, trials had to be prospective, controlled, involve participants diagnosed with cancer and test an exercise intervention. Types of outcome were not restricted. Two reviewers independently applied the selection criteria. RESULTS Thirty-three controlled trials (including 25 randomized trials) were included in the review. There was some evidence that physical function was increased among those who exercised. Furthermore, symptoms of fatigue did not appear to be increased and there were few adverse effects reported. There was insufficient evidence to determine effects on other outcomes, such as quality of life, with results hampered by the heterogeneity between studies as well as poor methodological quality. Data were also lacking on the long term effects of exercise relating to cancer recurrence or survival. CONCLUSIONS There is preliminary evidence that exercise interventions for cancer patients can lead to moderate increases in physical function and are not associated with increased symptoms of fatigue. However, it is impossible from current evidence to determine whether exercise has long term beneficial effects on survival or quality of life.