We sought to investigate the influence of interleukin-10 (IL-10) and IL-6 on the acute phase proteins (APP) in systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). IL-10, IL-6, Creactive protein (CRP), alpha-1-acid glycoprotein (AGP), and alpha1 antichymotrypsin (ACT) serum levels were determined in one hundred-eight patients (71 with SLE, 37 with RA). Quantification of the serum IL-10 level showed increased levels in SLE and RA patients as compared to healthy controls. Serum IL-6 level was found to be elevated in SLE and RA patients. A correlation between IL-10 and IL-6 serum level was found only in SLE. CRP and AGP serum levels were increased in RA as compared to controls, whereas in SLE only AGP was found elevated. A statistically significant correlation between IL-6 serum level and CRP, AGP and ACT was found only in RA. No correlation between IL-10 and serum level of CRP, AGP and ACT was established. Since IL-10 has a potent immunosuppressive activity, we expected it to be negatively correlated with APP levels. Surprisingly, IL-10 did not correlate with APP either in SLE or RA patients. However, the elevation of IL-10 serum levels in SLE and RA and the correlation between IL-10 and IL-6 in SLE may suggest that IL-10 may play a central role in inflammatory connective tissue diseases.