Previous research has shown a link between infectious inflammatory processes and hemostatic abnormalities. No data exist, however, on whether coagulation markers correlate with the severity of community-acquired pneumonia (CAP) at admission. We conducted a prospective, observational study in an Emergency Medicine Department of a primary care hospital. Sixty-eight patients admitted with CAP were included. Blood samples were collected at admission and assayed for D-dimer levels. D-dimers were correlated with the Pneumonia Patient Outcome Research Team (PORT) score and Acute Physiology and Chronic Health Evaluation II score on admission, with length of hospital stay, number of organ failures, time to defervescence and hospital mortality. D-dimer levels were positively correlated with the Acute Physiology and Chronic Health Evaluation II score (r = 0.44, P = 0.0002), the PORT score (r = 0.36, P = 0.002) and the length of hospital stay (r = 0.24, P = 0.046). Mean D-dimer levels of patients for whom hospitalization is recommended, according to PORT guidelines, were significantly higher than D-dimer levels of patients for whom hospitalization is not recommended (1.47 +/- 1.05 microg/ml and 0.71 +/- 0.79 microg/ml respectively; P = 0.006). The correlation between D-dimer levels and time to defervescence, development of organ system failure and outcome was not statistically significant. We conclude that D-dimer levels at admission may predict the severity of CAP.