In a previous clinical study it was found that patients with coronary heart disease and diabetics with peripheral artery disease often have an elevated erythrocyte aggregation value (AW) and that there is a positive correlation between AW and the number of risk factors found in a subject. In the present investigation we studied the relationship between AW and the serum concentration of high density lipoprotein cholesterol (HDL-C), which is known to be inversely associated with coronary heart disease (CHD) incidence. We found highly significant negative correlations between AW and HDL-C both in a subsample of the study population of a cross-sectional epidemiologic study on CHD risk factors (First Survey of the MONICA Project Augsburg) and in male patients with angiographically confirmed CHD. Correlation coefficients were -0.233 for normal men (P less than 0.01, n = 136), -0.261 for normal women (P less than 0.01, n = 117), and -0.745 for CHD patients (P less than 0.01, n = 14). The results support the concept that the erythrocyte aggregation value as an indicator of cardiovascular risk is consistent with established risk factor associations.