The assessment of body composition (BC) in morbidly obese patients is a difficult procedure. Air-displacement plethysmography (ADP), which measures body density, is a very promising technique for BC assessment in health and disease. However, there are very few data about the feasibility of applying ADP on morbidly obese patients, which theoretically could be affected by large body size and difficulty in lung volume measurements. The main aim of this pilot study was to evaluate the feasibility of using ADP for BC assessment in morbidly obese patients. We studied nine subjects (6 males and 3 females) who had a mean age (+/-SD) of 47.0+/-13.5 years and body mass index (BMI) of 46.6+/-7.7 kg/m(2) (range 36.4-58.8). All patients could fit into the instrument chamber and perform the manoeuvre for pulmonary plethysmography. Mean lung volume was 3.9+/-1.2 l and mean percent body fat was 53.1+/-6.6 (range 46.0-67.5). These results indicate that ADP appears to be suitable for patients with BMI over 40 kg/m(2) and produces realistic BC data.