We studied maximal aerobic work capacity in 24 cirrhotic patients whose clinical condition was well compensated. Nineteen of our patients had no heart or lung disease, and five had mild mechanical ventilatory impairment. The patients performed incremental exercise on a treadmill until subjective exhaustion. The maximal exercise levels reached by all were relatively low and led to a lower observed maximal oxygen uptake (VO2 max), than predicted uptake (19.6 +/- 0.5 vs. 37.9 +/- 0.6 ml/kg; p less than 0.001). Observed VO2 max values correlated strongly with the Pugh score, which reflects the degree of liver failure (r = -0.571; p less than 0.01). Since there were no clear cardiac or pulmonary causes to explain the decrease in work capacity, these observations suggest that liver cirrhosis might induce or be accompanied by muscular impairment. VO2 max, which seems to decline with the functional severity of the disease, may be a useful index for evaluating the capacity of patients for physical rehabilitation.