Correlations among ten diagnostic categories of hospital care physican performance measures are reported. Using measures of conformance to predetermined criteria for optimal performance, mean scores of individual physican performances within diagnostic categories were calculated and correlated. Measurement reliability estimates were computed and it was suggested that a minimum of four cases in each diagnostic category be used for measurement of performance on the individual physician level of analysis. There appeared to be homogeneity of performance measures among some but not all diagnoses studied. This finding reinforces the need to examine interdiagnostic correlations before attempting to measure overall individual physican performance by combining measures from separate diagnostic categories.