Background: Previous studies have demonstrated the presence of increased RBC aggregation and decreased RBC deformability in patients with acute coronary syndromes (ACS). However, the associations between biochemical markers and hemorheologic parameters remain unclear in patients with ACS and congestive heart failure (CHF). Methods: Between April 2008 and May 2010, 374 patients (243 men; mean age = 65.8 짹 13.4 years-old) admitted emergency room with chest pain or dyspnea were analyzed in this study. Critical-time and critical-shear stress for RBC aggregation and elongation index for RBC deformability were measured by automatic analyzers. Results: Of these patients, 67 patients were diagnosed of congestive heart failure and 250 patients were diagnosed of ACS. The critical shear-stress was significantly higher in patients with ACS compared to patients with atypical chest pain (214.7 짹 69.0 versus 192.6 짹 70.9, p = 0.031). There was no significant difference in RBC deformability between ACS patients and patients with atypical chest pain (0.3081 짹 0.0299 versus 0.3087 짹 0.0280, p=0.889). In CHF patients, there were no significant differences in hemorrheological parameters compared to patients with atypical chest pain. Among biomarkers, ESR (r = 0.207, p = 0.029) and C-reactive protein (r = 0.200, p = 0.013) were significantly correlated with critical-sheer stress in patients with ACS. There were no significant correlations between hemorrheological parameters and biomarkers in CHF patients. Conclusion: The biomarkers in ACS patients were associated with the hemorrhological parameters, whereas there were no significant associations in CHF patients. Further studies are required to determine the explanation for the association between biomarkers and hemorheological parameters in patients with various cardiovascular diseases.