Increasing the precision of measurements of total energy expenditure in population-based epidemiological studies is important for accurately quantifying the relationship between this exposure and disease. Current questionnaire-based methods cannot accurately quantify total energy expenditure, although they may provide an estimate of the frequency of vigorous activities. Heart rate monitoring with individual calibration has been advocated as a method for assessing energy expenditure in field studies and has been compared with the 'gold standard' techniques of doubly-labelled water and indirect calorimetry. However the method has previously only been used on small and selected populations. This study was, therefore, established to test the feasibility of using heart rate monitoring in a population-based study of adults. A total of 167 individuals aged 30-40 years were randomly selected and underwent 4 d heart-rate monitoring. Only three individuals could not complete the protocol. The mean physical activity level (PAL) measured over 4 d was 1.89 (SD 0.40) in men and 1.76 (sd 0.31) in women. There was no difference between mean PAL on weekend days compared with weekdays (mean paired difference 0.0008, 95% CI -0.06 +0.06). The estimate of mean PAL was not correlated with BMI, percentage body fat or the waist:hip ratio. It was, however, correlated with cardio-respiratory fitness as measured by VO2(max) per kg (Spearman rank correlation coefficient 0.50 in men and 0.42 in women). The pattern of energy expenditure was assessed by calculating the percentage of daytime hours in which PAL was greater than five times basal energy expenditure. This measure was strongly correlated with the mean PAL in both men (Spearman correlation coefficient 0.77) and women (0.71). We conclude that heart-rate monitoring is a feasible method for assessing the pattern and total level of energy expenditure in medium-sized epidemiological studies. It may also prove useful as the reference technique for calibrating questionnaires to estimate energy expenditure in larger scale studies.