National survey data show that reported energy intake has decreased in recent decades despite a rise in the prevalence of obesity. This disparity may be due to a secular increase in under-reporting or a quantitatively greater decrease in energy expenditure. This study examines the extent of under-reporting of energy intake in the National Diet and Nutrition Survey (NDNS) in young people aged 4-18 years in 1997 using published equations to calculate estimated energy requirements. It explores secular changes by comparison with the Diets of British School Children (DBSC) survey in 10-11- and 14-15-year-olds in 1983. In the NDNS, under-reporting (estimated energy requirements--energy intake) represented 21 % of energy needs in girls and 20 % in boys. The magnitude of under-reporting increased significantly with age (P<0.001) and was higher in overweight than lean individuals over 7 years of age. To compare reported energy intake in DBSC and NDNS, the estimated physical activity level from dietary records (dPAL=reported energy intake/predicted BMR) was calculated. If there were no under-reporting, dPAL would represent the subject's true activity level. However, dPAL from the NDNS was significantly lower than that from the DBSC by 8 % and 9 % in boys and girls for those aged 10-11 years, and by 14 % and 11 % for 14-15-year-olds respectively, reaching physiologically implausible levels in the 14-15-year-old girls (dPAL=1.17). If activity levels have remained constant between the two surveys, under-reporting has increased by 8-14 %. The evidence supports a secular trend towards increased under-reporting between the two surveys, but the precise magnitude cannot be quantified in the absence of historical measures of energy expenditure.