BACKGROUND In nine patients with multiple organ failure ultrasound was able to identify muscle wasting despite the presence of oedema (Campbell et al., J Clin Nutr 62 (1995) 533). OBJECTIVE The purpose of the present study was twofold: one was to determine whether this technique was applicable to a much larger ICU population, many of whom were not as ill as the original subjects. The second reason was to determine whether a relationship could be identified between rates of wasting and energy balance. METHODS Serial measurements of both mid-upper arm circumference (MAC) and muscle thickness, using ultrasound, were made at 1-3 day intervals between 5 and 39 (median 7) days in 50 critically ill patients. RESULTS Muscle thickness decreased in 48 of the 50 patients at a median rate of 1.6%/day with a range of 0.2-5.7%/day. In 33 patients, in whom MAC did not change significantly with time, muscle thickness decreased by between 0.3 and 4.2 (median 1.6)%/day. In three patients MAC increased significantly with time but muscle thickness decreased by between 1.3 and 5.7 (median 2.6)%/day. Twelve patients showed a significant decrease in MAC with time and muscle thickness in this group decreased by between 0.2 and 4.0 (median 1.3)%/day. The percentage decrease in muscle thickness between the groups, in whom MAC decreased or did not change, was not significantly different (P = 0.475). CONCLUSIONS We have demonstrated that an ultrasound technique devised to identify muscle wasting in the presence of severe fluid retention works in the majority (48/50) of patients when applied to a wider ICU population. Energy balance made no difference to the rate of wasting.