The erythrocyte aggregation value (AW) has been determined in groups of patients with and without coronary heart disease (CHD) and in a group of apparently normal subjects, using the Metricell apparatus as described by Kachel et al. The following results were obtained: There was a close correlation between the AW and the existence of CHD or arteriosclerosis of peripheral arteries. The precision of the retrospective prediction of CHD by this noninvasive test in male subjects aged over 45 years (total CHD prevalence = 0.494) was: sensitivity = 0.571, specificity = 0.767. The predictive value of positive tests (PVpos) was 0.706, of negative test (PVneg) 0.647. The discriminatory ability of the AW test was superior to that of serum cholesterol determinations or measurement of other CHD risk factors made in the same population. The AW rises exponentially with the total score for risk factors for CHD hazard for the subject. It appears, however, from the insignificant correlations observed that there is no direct connection between the AW and any single risk factor.