A small portable accelerometer was developed to estimate the energy expenditure of daily activities. The accelerometer is reported to be an improvement over movement counters currently on the market. The oxygen requirement of 14 different activities was measured in 21 subjects while each wore the accelerometer on the waist. A movement counter (mercury switch), which is available commercially, was also worn on the waist and another was worn on the left wrist. The reproducibility of the accelerometer readings was high (4 subjects, 14 activities; r = 0.94) and was superior to either the waist movement counter (r = 0.63) or the wrist movement counter (r = 0.74). In estimating oxygen requirement (VO2) the standard error of estimate, based on 21 subjects and 14 activities, was 6.6 ml X min-1 X kg-1 for the accelerometer. This was also better (smaller) than for the waist movement counter (9.2 ml X min-1 X kg-1) or for the wrist movement counter (7.9 ml X min-1 X kg-1).