Cigarette smoking is an established risk factor for cancer and cardiovascular disease, and is the leading cause of avoidable disease in most industrialized countries. Less well-known are possible beneficial effects, which are briefly considered in this survey. Preliminary data suggest that there may be inverse associations of smoking with uterine fibroids and endometriosis, and protective effects on hypertensive disorders and vomiting of pregnancy are likely. Smoking has consistently been found to be inversely related to the risk of endometrial cancer, but cancers of the breast and colon seem unrelated to smoking. Inverse associations with venous thrombosis and fatality after myocardial infarction are probably not causal, but indications of benefits with regard to recurrent aphthous ulcers, ulcerative colitis, and control of body weight may well reflect a genuine benefit. Evidence is growing that cigarette smoking and nicotine may prevent or ameliorate Parkinson's disease, and could do so in Alzheimer's dementia. A variety of mechanisms for potentially beneficial effects of smoking have been proposed, but three predominate: the 'anti-estrogenic effect' of smoking; alterations in prostaglandin production; and stimulation of nicotinic cholinergic receptors in the central nervous system. Even established inverse associations cannot be used as a rationale for cigarette smoking. These data can be used, however, to clarify mechanisms of disease, and point to productive treatment or preventive options with more narrowly-acting interventions.