Objectives: Several instruments for measuring health-related quality of life (HRQOL) have been developed, and others are under construction. The problem is whether the different HRQOL measures show comparable results. We first compared the functional relationship of the Nottingham Health Profile (NHP) and the 15-dimensional measure of HRQOL (15D) in hip and knee replacement patients. The hypothesis was that condition or intervention does not affect the functional relationship between NHP and 15D changes.

Methods: We assessed the agreement of the instruments by comparing observed changes in the 15D and its fitted values derived by regressing the 15D by the NHP dimensions. Patients (n = 452) were recruited consecutively from seven Finnish orthopedic departments during April 1991–May 1992. HRQOL was measured prior to surgery and 6, 12, and 24 months postoperatively.

Results: There was a different functional relationship between the HRQOL instruments in hip and knee patients; they agreed upon the direction of changes in HRQOL in 84% and showed opposite signs in 12%. The NHP showed significantly more improvement in quality of life than the 15D.

Conclusions: The two instruments were in slight but significant disagreement. Estimates of effectiveness can vary according to the instrument used. Comparisons of effectiveness in healthcare programs measured by differing instruments need information on the functional discrepancies between the instruments in the conditions and interventions in which they are applied.