Total daily energy expenditure (TEE) by the doubly-labelled (2H218O) water method and basal metabolic rate (BMR) by indirect calorimetry were measured in thirty-two healthy free-living adults in Northern Ireland. Habitual physical activity patterns in occupational and discretionary activities were assessed by interview questionnaire. Expressed as a multiple of BMR the TEE values for the sixteen males (1.88 (sd 0.28), range 1.44–2.57) and sixteen females (1.77 (sd 0.16), range 1.50–2.06) were compatible with current Department of Health and Social Security (DHSS; 1979) and Food and Agriculture Organization/World Health Organization/United Nations University (FAO/WHO/UNU; 1985) estimates of energy requirements. The results suggest that discretionary physical activity is now emerging as an equally important determinant of energy expenditure in the UK as the occupational classifications currently used as the basis of DHSS (1979) and FAO/WHO/UNU (1985) recommendations for energy requirements. Therefore, realistically achievable inputs of recreational exercise can have a significant impact in counteracting low levels of energy expenditure which are associated with modern lifestyles and are implicated as a risk factor for coronary heart disease and obesity.