Cigarette smoking and the incidence of Parkinson's disease in two prospective studies

An inverse association between cigarette smoking and idiopathic Parkinson's disease has been reported in several retrospective studies, but prospectiv e evidence is available only for men. We assessed the association between t he incidence of Parkinson's disease and smoking in two large prospective co hort studies comprising men and women. New cases of Parkinson's disease wer e identified in the Nurses' Health Study for 1976-1996, and in the Health P rofessionals Follow-up Study for 1986-1996. Smoking history was assessed at baseline and updated on subsequent biennial questionnaires. In women, the age-adjusted rate ratios (95% confidence intervals) for Parkinson's disease relative to never-smokers were 0.7 (0.5, 1.0) for past smokers, and 0.4 (0 .2, 0.7) for current smokers. In men, the agc-adjusted rate ratios for Park inson's disease relative to never-smokers were 0.5 (0.4, 0.7) for past smok ers, and 0.3 (0.1, 0.8) for current smokers. In both cohorts, the strength of the association decreased with time since quitting (among past smokers), increased with number of cigarettes per day (among current smokers), and i ncreased with pack-years of smoking. These prospective findings confirm tha t an inverse association between smoking and the incidence of Parkinson's d isease exists in both men and women.