In order to examine the effect of corticosteroids on coronary atherogenesis in collagen diseases, an experimental study of serum sickness was performed. Forty-two rabbits were divided into four groups (Groups A-D). Group B, C and D rabbits received four intravenous injections of bovine serum albumin (250 mg/Kg) at 16-day intervals. Groups A, C and D rabbits were fed ad libitum cholesterol supplemented diet (1%) 16 days after the last injection. Group D rabbits received subdermal injections of prednisolone (1 mg/Kg) three times per week in the same period. After 124 days, all rabbits were sacrificed. Serum cholesterol and phospholipid increased in Group A, C and D rabbits. Group A rabbits showed intimal foam cell proliferation. Group B rabbits showed slight fibrous intimal thickening. The coronary arteries of Group C rabbits showed fatty-proliferative intimal thickening and an increase in the incidence of vascular lesions (13.9% of the coronary arteries as compared with 11.7% for Group A and 8.4% for Group B). The coronary lesions of Group D showed the same pattern as those of Group C, but the incidence of lesions was 6.0%. It was concluded that prednisolone did not augment immunologically induced atherosclerosis.