The state of leukocyte and erythrocyte adhesiveness/aggregation was determined in the peripheral blood of 382 patients with infection/inflammation as well as in 72 controls by using a simple slide test and image analysis. A highly significant correlation (r = 0.4, n = 455, p < 0.001) was found between the state of leukocyte and erythrocyte adhesiveness/aggregation. The extent of both leukocyte and erythrocyte aggregation correlated with the concentration of fibrinogen. Significant aggregation of leukocytes with erythrocytes was noted as well. We conclude that both leukocyte and erythrocyte aggregation occur in the peripheral blood of patients with infection/inflammation. Such cell aggregation, which might have detrimental rheological consequences, can be detected by using our novel technique.