In order to study the role of interleukin-6 (IL-6) in inflammatory disease we monitored plasma levels of IL-6 and acute phase proteins such as C-reactive protein (CRP) and renin substrate (RS) in patients with reactive arthritis (ReA), rheumatoid arthritis (RA), and systemic lupus erythematosus (SLE). Venous plasma samples were collected: (1) during the acute phase or exacerbation of the disease, and (2) several months latter during convalescence. Increased mean [95% confidence intervals (CI)] levels of plasma IL-6 were observed in patients with ReA both in the acute phase and later, 229 (177 to 280) ng/l and 197 (134 to 260) ng/l respectively (P less than 0.001 as compared to controls). The corresponding plasma IL-6 levels in RA patients were 283 (223 to 340) ng/l and 183 (151 to 226) ng/l, respectively (P less than 0.001 as compared to controls). Plasma IL-6 levels in SLE patients were not increased. Plasma RS levels were increased in all patient groups, but no significant correlation to IL-6 or CRP levels was observed, whereas plasma IL-6 and CRP levels showed a positive correlation in ReA and RA patients.