An arthritis-specific health index (ASHI) for the SF-36 Health Survey was developed by studying its responsiveness to changes in clinical indicators of arthritis severity. Longitudinal data from 1,076 patients participating in four placebo-controlled trials were analyzed. All had at least a 6-month history of moderate to severe osteoarthritis or rheumatoid arthritis of the knee or hip. All had undergone a washout period of 3 to 14 days before baseline assessment to bring about a flare state in osteoarthritis or rheumatoid arthritis symptoms. Their average age was 60 years and 72% were female. Change scores for the eight-scale SF-36 health profile (acute version) and five arthritis-specific measures of disease severity (knee pain on weight bearing, time to walk 50 feet, physician global evaluation of symptom severity and impact, patient global evaluation of symptom severity and impact, and pain intensity visual analogue scale) were computed by subtracting scores before treatment from scores at two-week follow-up. Canonical correlation methods were used to derive weights for changes in SF-36 scales to score a single index (ASHI) that maximized its correlation with changes in the set of five clinical measures of arthritis severity. The weights used to score the ASHI were cross-validated in a 25% holdout group (N = 144) from the first two osteoarthritis trials and in two additional osteoarthritis and rheumatoid arthritis trials (N = 530). Only one SF-36 canonical variate (ASHI) correlated significantly (F = 4.69, P < 0.0001) with the clinical canonical variate that served as the "criterion" measure of change in the severity of arthritis. Changes in the ASHI and clinical canonical variate were substantially correlated in the developmental sample (r = 0.628, P < 0.0001) and on cross-validation (r = 0.629, P < 0.0001). The clinical canonical variate correlated highly (r = 0.75-0.88) with changes in all but one of the five clinical measures (50-foot walk; r = 0.41). The pattern of correlations between changes in SF-36 scales and the ASHI indicated that ASHI is primarily a measure of bodily pain (r = 0.92) and other aspects of physical and role functioning and well-being (r = 0.69 for Role-Physical, r = 0.68 for Physical Functioning, r = 0.52 for Social Functioning, and r = 0.51 Vitality). The patterns of correlations between SF-36 scales and the ASHI were very similar across developmental and cross-validation samples. This research demonstrates the feasibility and generalizability of a single ASHI scored from changes in responses to the SF-36 Health Survey. The generic SF-36 health profile, which has already been shown to be useful in comparing arthritis with other diseases and treatments, can also be scored specifically to make it more useful in studies of osteoarthritis and rheumatoid arthritis.