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The association of obesity with low socioeconomic origin and downward social mobility raises the question if obesity is related to intellectual performance. This was investigated in the population of Danish young men who underwent obligatory draft board examination including measurement of height and weight and intelligence testing in Copenhagen and the surrounding provincial area during the period 1956-1977. We identified all with extreme overweight defined as a body mass index (weight/height2) of at least 31 kg/m2 (N=1806). For comparison we selected a random 1% sample (N=2719) of the remaining population. The test score in the obese group was significantly lower than in the control group; the difference between the mean values was 41% of the standard deviation in the control group, and 67% of the obese scored less than the median score of the controls. The test score was not significantly related to body mass index up to the limit of extreme overweight, but decreased with increasing index above that limit. The difference in test score between the two groups was consistent throughout the years of examination in both the metropolitan and provincial area. We conclude that severe obesity is strongly associated with reduced intellectual performance.