The Severity of Renal Disease Scale (SORDS) was developed to provide a single score reflecting disease severity of renal patients independent of confounding psychosocial influences. This study examined SORDS' reliability and validity and its relevance as a research tool assessing the psychological effect of illness severity. Data was collected from 127 renal patients (predialysis, HD, CAPD). SORDS was compared with the Endstage Renal Disease Severity Index (ESRD-SI), the SF-36, the Beck Depression Inventory - 2nd Edition and a subset of BDI-II items reflecting cognitive features only at differing stages of renal disease and time on dialysis. SORDS and ESRD-SI data from twenty-two CAPD patients was included in reliability analyses. SORDS reliability estimates were low suggesting that the use of SORDS with medical chart data at this time is problematic. SORDS should be used only by medical practitioners who are aware of patients' standing on SORDS variables. There was however strong support for SORDS' validity. Validity was demonstrated by correlations between SORDS and the ESRD-SI. Compared to the ESRD-SI, SORDS was better able to discriminate between dialysis and pre-dialysis patients. SORDS and ESRD-SI scores were related to self-perceptions of decreased health status on the SF-36 independent of dialysis duration and age. SORDS utility in psychosocial research with renal patients was demonstrated by a finding that disease severity differentially impacts levels of depression for HD versus CAPD patients independent of age or dialysis duration. At the lowest level of illness severity as assessed by SORDS, CAPD patients scored in the moderate range of depression and were significantly more depressed than HD patients. Using the same analyses but with the ESRD-SI, no differences in level of depression were detected. These results imply a relationship between adjustment to treatment and illness severity. It is concluded that SORDS is a valid index of renal disease severity and that illness severity as assessed by SORDS may have an important role as a moderator variable in psychosocial research with renal patients. These results may have important implications for treatment assignment and psychosocial assessment and intervention of renal patients and their families.