The relationship between smoking and neurological diseases has always been controversial. Even the expected association between smoking and increased risk for cerebrovascular disease has been debated for years. It was at the end of the 1980s that smoking became definitively accepted as a risk factor for ischemic stroke. More recently, two other neurological diseases have been studied in relation to smoking: Parkinson's disease (PD) and Alzheimer's disease (AD). Many epidemiological studies have found a highly significant negative association between cigarette smoking and these two neurodegenerative disorders. The risk of AD or PD in nonsmokers has generally been about twice that of smokers. That is, patients with AD or PD are approximately 50% less likely to have smoked cigarettes during their lifetime than are age- and gender-matched controls. Alternatively, cigarette smokers are 50% less likely to have PD or AD than are age- and gender-matched nonsmokers. This statistically significant negative association has been interpreted as suggesting that cigarette smoking exerts an undefined, biologic, neuroprotective influence against the development of PD and AD. A review of all studies that either support or refute this hypothesis is presented separately for PD and AD.