The aim of this study was to determine whether variations in physiological characteristics were predictive of over- or underreporting of energy intake in older men and women. Total energy expenditure was predicted in 81 older men (66 +/- 6 y) and 56 older women (66 +/- 6 y) from a previously developed equation that uses the doubly labeled water method. The degree of over- or underreporting of energy intake was derived by subtracting predicted total daily energy expenditure from self-reported energy intake obtained from a 3-d diet record. Older women underreported energy intake (-2206 kJ/d +/- 1841) to a greater degree than did older men (-1301 kJ/d +/- 1908) (P < or = 0.01). In men, none of the physiological variables were significantly correlated with over- or underreporting of energy intake. Among the women, percent body fat was negatively correlated with underreporting of energy intake (r = -0.42, P = 0.001). Thus, the underreporting of energy intake increased with the amount of adiposity in women.