Data from an ongoing Dutch health examination monitoring project carried out in 1995 (n = 2,079 men and 2,467 women, aged 20-65 years) were used to study whether various determinants of underreporting of energy intake influenced the association between underreporting and body mass index. Further, the authors examined whether these determinants were mutually independent predictors of underreporting. As a measure for the degree of underreporting, they calculated energy ratios of reported daily energy intake divided by the estimated basal metabolic rate. They observed that underreporting occurred more with increasing degrees of overweight in men and women. Each increase in body mass index by 1 kg/m2 was associated with a decrease in reported energy intake/basal metabolic rate (in men, beta = -0.0364; standard error, 0.0024; in women, beta = -0.0262; standard error, 0.0018). After adjustment for age, education, smoking habits, physical activity, dieting behavior, and dieting frequency during the last year, the slopes were reduced by 29% in men and 17% in women but remained negative and highly statistically significant. Adjustment for current dieting behavior particularly decreased the association between body mass index and underreporting. Age was another independent determinant of underreporting in men and women and, in men only, so were smoking habits and education level. In conclusion, overweight individuals give biased dietary information, and this may distort the relations between self-reported dietary intake and diseases related to body mass index.