Abstract: This study aims at evaluating on-admission serum level of d-dimer in patients with community-acquired pneumonia concerning the severity of the disease and in-hospital outcome of the patients. Sixty patients with community-acquired pneumonia were studied during a one-year period in Imam Khomeini and Sina Hospitals, Tabriz, Iran. On-admission serum d-dimer was measured by enzyme-linked immunoabsorbent assay and the severity of disease determined according to PORT grading system. In-hospital outcome was determined in regard to the level of serum d-dimer. Sixty patients with community-acquired pneumonia, 39 males and 21 females were enrolled. There were twelve patients with PORT one, eight patients with PORT two, eight patients with PORT three, twenty patients with PORT four and twelve patients with PORT five. The mean level of serum d-dimer was significantly higher in severe disease (p<0.001), patients with hospital stay longer than one week (p = 0.003), patients with bronchopulmonary pattern (p = 0.012), cases in-need of mechanical ventilation (p<0.001) and patients who expired during hospital stay (p = 0.022). On-admission level of serum d-dimer was significantly and independently higher in patients with severe disease (p<0.001) and in cases with bronchopulmonary pattern on chest x-ray (p = 0.035). On-admission level of serum d-dimer may predict the severity of community-acquired pneumonia. Further studies are recommended for accurate cut-off points.