To elucidate the characteristics of measures of function in patients with chronic heart failure and chronic lung disease we administered four functional status questionnaires, a 6-min walk test and a cycle ergometer exercise test, to 43 patients limited in their day to day activities as a result of their underlying heart or lung disease. Correlations between these measures were calculated using Spearman's rank order correlation coefficient. The walk test correlated well with the cycle ergometer (r = 0.579), and almost as well with the four functional status questionnaires (r = 0.473-0.590) as the questionnaires did with one another (0.423-0.729). On the other hand, correlations between cycle ergometer results and the questionnaires was in each case 0.295 or lower, and none of these correlations reached statistical significance. These results suggest that exercise capacity in the laboratory can be differentiated from functional exercise capacity (the ability to undertake physically demanding activities of daily living) and that the walk test provides a good measure of function in patients with heart and lung disease.