In this study, we compared the relationships of body mass index (BMI) and body adiposity index (BAI) with body fat percentage (BF%) in a Caucasian, European population. BF% was measured by dual-energy x-ray absorptiometry in a population-based cross-sectional study of 5,193 middle-aged (47–49 years) and elderly (71–74 years) men and women from the Hordaland Health Study in western Norway from 1997 to 1999. In the total population, the correlation between BAI and BF% was stronger (r = 0.78) than the correlation between BMI and BF% (r = 0.56) with similar results in the middle-aged and elderly groups. However, in men and women separately, BMI was a better correlate of BF% (for men, r = 0.76; for women, r = 0.81) than was BAI (for men, r = 0.57; for women, r = 0.72). BMI was also a better correlate of BF% than was BAI assessed by partial correlations adjusted for sex (for BMI-BF%, r = 0.79; for BAI-BF%, r = 0.67). Bland-Altman plots and BF%-stratified analyses showed that BAI tended to overestimate BF% in lean subjects and to underestimate it in those with higher proportions of body fat, but that it predicted BF% well for those whose BMI was in a normal range. At the individual level and in population studies adjusted for sex, BMI outperforms BAI as a predictor of BF%.