We studied the validity of a generic health measure in a population with a chronic, life-shortening illness. Thirty-seven adults with cystic fibrosis (CF) and 46 of their healthy peers completed a questionnaire which included 12 questions on functional status from the RAND Health Insurance Study. For the CF group, the questionnaire and a medical chart review yielded data on 7 additional health variables, including pulmonary function. After data collection, members of the CF group were followed for 5 years, by which time 11 had died. The functional status of the CF group was significantly lower than that of the comparison group. Within the CF group, functional status correlated significantly with 6 of the 7 other health variables. Analysis using the Cox proportional hazards model showed that functional status alone was a significant (p less than 0.001) predictor of a CF subject's survival time; in a multivariate model a non-significant trend suggested that lowered functional status may be associated with an increased risk of early death even after adjustment for pulmonary function and percent ideal body weight. These results extend previous findings and suggest that functional status can be used as an overall measure of health in a wide variety of studies.