OBJECTIVE: To determine the contribution of genetic factors to cognitive functioning in older men. DESIGN: Cognitive testing by telephone interview in an epidemiologically defined population. PARTICIPANTS: 2077 monozygotic and 2225 dizygotic male twin pairs, all between the ages of 62 and 73 years, recruited from the National Academy of Sciences twin registry. MAIN OUTCOME MEASURES: The Telephone Interview for Cognitive Status--Modified total score and factor scores were analyzed. The Falconer heritability statistic and maximum likelihood estimates of genetic and environmental components were computed. RESULTS: Heritability of the total Telephone Interview for Cognitive Status--Modified score was estimated to be 30%. Shared environmental effects accounted for an additional 18% of the variance; most of this was related to years of education. Of the four cognitive factors derived, the language/attention factor had the highest heritability estimate. CONCLUSIONS: Genetic factors and educational achievement together account for almost half of the variance in the cognitive functioning of older men. Studies of the genetics of dementing illnesses need to consider the degree to which cognitive capacities are themselves under genetic control.