This review presents the available evidence on the health effects of reduced smoking. Smoking reduction was defined as reduction of the daily intake of tobacco without quitting. Only published papers were reviewed. Case reports and studies without a thorough definition of smoking reduction or health outcome were excluded. We searched in personal databases, BioMail Medline Search, Medline, the Cochrane Database of Systematic Reviews, and EMBASE. We followed the QUORUM standards for systematic reviews, and both authors read and discussed all publications. A total of 25 studies (31 publications) were identified: 8 articles reported on effects on the cardiovascular system; 11 on the airways; 7 on carcinogens, DNA damage, and lung cancer; 3 on birth weight; and 4 on other health effects. Some papers assessed more than one outcome. In most studies, reduction was defined as less than 50% of baseline tobacco consumption. Most of the studies were small, with the populations selected and short follow-up periods. The limited data suggest that a substantial reduction in smoking improves several cardiovascular risk factors and respiratory symptoms. In addition, smoking reduction is associated with a 25% decline in biomarkers and incidence of lung cancer and a small, mostly nonsignificant, increase in birth weight. There seem to be no substantial beneficial effects on lung function. The evidence on other health effects and mortality is too limited to draw conclusions. A substantial reduction in smoking seems to have a small health benefit, but more studies are needed to determine the long-term effects of smoking reduction.