Background: Thrombocytosis, often considered a marker of normal inflammatory reaction of infections, has been recently associated with increased mortality in hospitalized patients with communityacquired pneumonia (CAP). We assessed the characteristics and outcomes of patients with CAP and thrombocytosis (platelet count ≥ 4 × 105/mm3) compared with thrombocytopenia (platelet count, 105/mm3) and normal platelet count. Methods: We prospectively analyzed 2,423 consecutive, hospitalized patients with CAP. We excluded patients with immunosuppression, neoplasm, active TB, or hematologic disease. Results: Fifty-three patients (2%) presented with thrombocytopenia, 204 (8%) with thrombocytosis, and 2,166 (90%) had normal platelet counts. Patients with thrombocytosis were younger (P < .001); those with thrombocytopenia more frequently had chronic heart and liver disease (P < .001 for both). Patients with thrombocytosis presented more frequently with respiratory complications, such as complicated pleural effusion and empyema (P < .001), whereas those with thrombocytopenia presented more often with severe sepsis (P < .001), septic shock (P = .009), need for invasive mechanical ventilation (P < .001), and ICU admission (P = .011). Patients with thrombocytosis and patients with thrombocytopenia had longer hospital stays (P = .004), and higher 30-day mortality (P = .001) and readmission rates (P = .011) than those with normal platelet counts. Multivariate analysis confirmed a significant association between thrombocytosis and 30-day mortality (OR, 2.720; 95% CI, 1.589-4.657; P < .001). Adding thrombocytosis to the confusion, respiratory rate, and BP plus age ≥65 years score slightly improved the accuracy to predict mortality (area under the receiver operating characteristic curve increased from 0.634 to 0.654, P = .049). Conclusions: Thrombocytosis in patients with CAP is associated with poor outcome, complicated pleural effusion, and empyema. The presence of thrombocytosis in CAP should encourage ruling out respiratory complication and could be considered for severity evaluation. © 2013 American College of Chest Physicians.