BACKGROUND: Physical activity is recommended for secondary prevention of several diseases but it is not always clear how reliable the evidence is. METHODS: We searched Medline and the Cochrane Library for systematic reviews of randomised clinical trials published 1998 to 2004. RESULTS: We identified 30 eligible systematic reviews and excluded 13 that contained trials covered in larger reviews or were older than other reviews on the same subject. Physical activity decreased all-cause mortality in patients with coronary heart disease, odds ratio 0.73 (95% confidence interval 0.54 to 0.98), increased maximum walking time in patients with intermittent claudication by 6.5 min (4.4 to 8.7), and decreased pain in patients with osteoarthritis of the knee, standardised mean difference 0.34 (0.24 to 0.44). There were positive effects also in heart failure, chronic obstructive lung disease, type 2 diabetes and fibromyalgia, but they need confirmation in high-quality trials. Exercise improved quality of life in several conditions and generally led to improved physical performance. An effect was not shown in stroke, asthma, rheumatoid arthritis, acute or chronic low back pain, chronic fatigue syndrome, depression, cystic fibrosis or HIV/AIDS. The occurrence of harms was generally not reported. CONCLUSION: Physical activity can have important, and even life-saving, effects as secondary prevention of disease, but more and better trials are needed to fully assess its benefits and harms, in particular trials that compare exercise with drugs.

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