The availability of biomarkers to evaluate the risk of cardiovascular diseases is limited(1). High fibrinogen levels have been identified as a relevant cardiovascular risk factor, but the biological mechanisms remain unclear(2,3). Increased aggregation of erythrocytes (red blood cells) has been linked to high plasma fibrinogen concentration(2,4). Here, we show, using atomic force microscopy, that the interaction between fibrinogen and erythrocytes is modified in chronic heart failure patients. Ischaemic patients showed increased fibrinogen-erythrocyte binding forces compared with non-ischaemic patients. Cell stiffness in both patient groups was also altered. A 12-month followup shows that patients with higher fibrinogen-erythrocyte binding forces initially were subsequently hospitalized more frequently. Our results show that atomic force microscopy can be a promising tool to identify patients with increased risk for cardiovascular diseases.