Background: Drugs are heavily promoted for primary prevention of disease whereas possible benefits of physical activity have received less attention. We have studied whether there is reliable evidence that exercise is effective. Methods: We searched MEDLINE and The Cochrane Library for systematic reviews of randomised clinical trials published 1998-2004. Results: We identified 31 eligible reviews and excluded 20 that contained trials covered in larger reviews or were older than other reviews on the same subject. The 11 included reviews comprised altogether 252 trials and 16,179 participants. Generally, the included trials were of poor quality. The most reliable result was that exercise in the elderly can reduce falls resulting in injury, relative risk 0.7 (95% confidence interval 0.5 to 0.9), although this effect could be due to modification of risk factors. Other results were a reduction in systolic blood pressure by 3.8 mmHg (2.7 to 5.0) and diastolic blood pressure by 2.6 mmHg (1.8 to 3.4), beneficial changes in blood lipids, a positive effect on bone mineral density in the spine of 1.8% (0.6% to 3.0%), and a lack of effect on smoking cessation. The reporting of harms of exercise was virtually non-existent. Conclusions: Exercise can have important benefits. There is a need for large trials that live up to accepted standards and include measurement of harms, in particular for trials that compare exercise with drugs.