Interleukin-1-beta (IL-1β has been implicated as an inflammatory mediator in rheumatoid arthritis (RA) but little is known about the related cytokine, IL-1α, in this disease. IL-1α has biological properties similar to IL-1β but, unlike IL-1β remains mostly cell-associated. In this study plasma IL-α was measured by radioimmunoassay in patients with RA and in healthy controls. Plasma levels were compared with conventional measures of disease activity. The mean levels in the two groups were not significantly different and, within the patient group (n = 53), the only significant cross-sectional correlation was between plasma IL-1α and ESR. In longitudinal studies, some individual patients had plasma IL-1α levels that correlated with different measures of disease activity. We conclude that while IL-α may be involved in the immunopathogenesis of RA, its measurement in plasma seems to offer little of clinical value.