AIM:To systematically review the possible mechanisms by which maternal smoking affects lactation. METHODS:Databases (Medline, CINAHL, Current Contents, Psychinfo, Sociological Abstracts and the Cochrane Library) were searched for smoking and breastfeeding or infant feeding. The Journal of Human Lactation and Birth were hand-searched, searches were also conducted at NMAA's Lactation Resource Centre and references cited were located where appropriate. Articles were divided into the various ways that smoking could affect breastfeeding and were tabulated. RESULTS:Most studies were conducted on small samples of animals or humans; the majority were prior to 1985. Most animal studies exposed the animals to much greater levels of nicotine than those to which humans are exposed. Most studies did not examine if breastfeeding behaviour was similar in smokers and non-smokers, and did not consider that any physiological difference found could be the result of poor lactation practices, rather than the cause of poor lactation in smokers. The definition of breastfeeding infants was also problematic in many studies. The effect of smoking on oxytocin in women was only examined in one study, and no effect was found, yet a negative effect of smoking on oxytocin release is reported in the breastfeeding literature. CONCLUSION:Although there is consistent evidence that women who smoke breastfeed their infants for a shorter duration than non-smokers, the evidence for a physiological mechanism is not strong.