Measures of disease severity are required as experimental, control and outcome variables in studies of the psychosocial aspects of physical illness. The Endstage Renal Disease Severity Index (ESRD-SI) was developed for these purposes during a prospective study of patient adjustment to dialysis and endstage renal disease (ESRD). It was found to have adequate inter-rater reliability, test-retest reliability, and construct and predictive validity in two subsamples (total N = 233) of dialysis patients. The ESRD-SI is suitable as a sensitive indicator of disease severity in these patients.