Osteoprotegerin (OPG) is a member of the tumour necrosis factor superfamily involved in the regulation of bone metabolism and vascular calcification. High serum values of OPG are associated with cardiovascular disease in humans. The purpose was to investigate serum OPG levels in subclinical carotid atherosclerosis and the relation between OPG levels and plaque morphology. OPG levels were compared in 29 persons with echogenic carotid plaques, 30 persons with echolucent plaques and 41 persons without carotid plaques, all recruited from a population health study. Computerized assessment of plaque echogenicity was done by use of the gray scale median (GSM). Participants with echogenic carotid plaques had lower serum OPG level (1.23 ng/ml; 1.02-1.48) (geometric mean; 95% CI) than persons with echolucent plaques (1.76 ng/ml; 1.46-2.14) and those without plaques (1.89 ng/ml; 1.60-2.21). OPG and PTH were independently related to GSM. A significant trend for decrease in GSM across quartiles of OPG was found (p=0.003) which remained significant even after adjustment for PTH and smoking. The present study demonstrates lower serum OPG levels in persons with subclinical echogenic carotid plaques and identified an inverse relation between serum OPG and plaque echogenicity. The findings support the concept that OPG may play an important role in arterial calcification.