Objective: To assess the strength of the effect of cardiovascular risk factors on the incidence of giant cell arteritis (GCA) in a general population context. Method: Data from the Reykjavik Study (RS), a population-based cohort study focusing on cardiovascular disease, were used. Everyone born in 1907-1935 living in Reykjavik, Iceland, or adjacent communities on 1 December 1967 were invited to participate. Subjects attended a study visit in 1967-1996 and information on cardiovascular risk factors smoking habits, blood pressure, diabetes, body mass index (BMI), and serum cholesterol was obtained. All temporal artery biopsies obtained from members of the RS cohort were re-examined by a single pathologist with expertise in vascular pathology. Effects of risk factors on GCA occurrence are expressed as incidence rate ratios (IRRs) with 95% confidence intervals (CIs). Results: Altogether, 19 241 subjects contributed a median of 23.1 (interquartile range 17.6-29.4) years after the age of 50 to this analysis. During 444 126 person-years of follow-up, 194 subjects developed GCA, corresponding to an incidence rate of 43.6 (95% CI 37.8-50.2) per 100 000 person-years. Being overweight or obese  were inversely associated with GCA, especially in women IRRs 0.70 (0.48-1.02) and 0.31 (0.14-0.71), respectively. There was a weaker association between BMI and incident GCA in men. Smoking was inversely associated with GCA in men IRR 0.47 (0.27-0.81), but not in women. Conclusions: The incidence of GCA in Iceland is very high. High BMI protects against the occurrence of GCA, and smoking may protect against GCA in men.