OBJECTIVE: To investigate whether the serum levels of IL-10 in patients with rheumatoid arthritis are different from those of normal controls and SLE patients and to find out any correlation with disease activity parameters of rheumatoid arthritis. METHODS: Sera from 20 healthy normal persons, 16 rheumatoid arthritis patients and 16 patients with systemic lupus erythematosus were collected and measured for IL-10 and IL-6. Various disease activity parameters were measured in RA patients. RESULTS: The serum level of IL-10 in RA patients was significantly elevated compared to normal controls but lower than those of SLE patients. In RA patients there was no definite correlation between the disease activity parameters and serum IL-10 levels. Despite significant improvements in terms of various disease activity parameters, there was no significant change of serum IL-10 levels after treatment in RA patients. In seropositive RA patients, positive correlation was found between serun IL-10 and rheumatoid factor levels. CONCLUSION: Our findings show that the serum IL-10 levels in patietns with RA are elevated compared to normal controls but lower than those of SLE patients. There was no correlation between serum IL-10 levels and disease acivity parameters of RA.