We investigated whether serum titers of interleukin 10 (IL-10), a cytokine known to shift lymphocyte responses towards humoral immunity, reflect disease activity in systemic lupus erythematosus (SLE). Sera from 72 SLE patients, 25 RA patients and 30 healthy controls were tested for IL-10 by ELISA. Low titers of IL-10 were detected in the serum of 37.5% of SLE patients and in 24% of RA patients but in only 3% of healthy controls. Interestingly, serum IL-10 titers in SLE patients were positively correlated with the SLE Disease Activity Index (SLEDAI) and with anti-DNA antibody titers, but negatively with complement fraction C3 levels. These results indicate that serum IL-10 values reflect SLE disease activity and suggest that overexpression of IL-10 might play a pathogenic role in severe lupus disease.