Inflammatory and lipid factors share an important role in atherosclerosis. This study evaluates their relations in dyslipidemic subjects.We compared the complement system (serum hemolytic activity CH50, C3 and C4 fractions and terminal complex sC5b-9) in 30 hypercholesterolemic patients with elevated cholesterol and decreased HDL-cholesterol levels, 30 normolipemic patients with clinical atherosclerosis and 30 matched normal subjects. In addition we evaluated the circulating immune complexes containing cholesterol (chol-CIC) on the assumption that they might be important in complement activation, and the circulating levels of the adhesion molecule ICAM-1 (sICAM-1) as a sign of endhotelial dysfunction. We found a significant increase of sC5b-9 (but not of CH50 and C3, C4) in the hypercholesterolemics compared with the other groups. The plasma sC5b-9 level was inversely and significantly related to HDL-chol (regression analysis), whereas no direct significant relation was found between sC5b-9 and cholesterol. Chol-CIC were also significantly increased in this group. The atherosclerosis patients also presented a significant increase of sC5b-9. Lastly, both patient groups displayed a significant increase of sICAM-1.We suggest that complement activation in dyslipidemics may be induced by their increased immune complexes. However, the decrease of complement regulatory proteins carried by HDL is another important factor, while complement changes may be related to variations of other humoral and cell systems (endothelium, coagulative/fibrinolytic system), whose involvement is suggested in our study by the changes of sICAM-1.