Background: Little is known about the clinical presentation and outcome of pneumococcal lower respiratory tract infection (LRTI) without positive chest X-ray findings and blood cultures. We investigated the prognostic impact of a pulmonary infiltrate and bacteraemia on the clinical course of hospitalized patients with confirmed pneumococcal LRTI.Methods: We studied a population-based multi-centre cohort of 705 adults hospitalized with LRTI and Streptococcus pneumoniae in LRT specimens or blood: 193 without pulmonary infiltrate or bacteraemia, 250 with X-ray confirmed pneumonia, and 262 with bacteraemia. We compared adverse outcomes in the three groups and used multiple regression analyses to adjust for differences in age, sex, comorbidity, and lifestyle factors.Results: Patients with no infiltrate and no bacteraemia were of similar age but had more comorbidity than the other groups (Charlson index score ≥1: no infiltrate and no bacteraemia 81% vs. infiltrate without bacteraemia 72% vs. bacteraemia 61%), smoked more tobacco, and had more respiratory symptoms. In contrast, patients with a pulmonary infiltrate or bacteraemia had more inflammation (median C-reactive protein: no infiltrate and no bacteraemia 82 mg/L vs. infiltrate without bacteraemia 163 mg/L vs. bacteraemia 316 mg/L) and higher acute disease severity scores. All adverse outcomes increased from patients with no infiltrate and no bacteraemia to those with an infiltrate and to those with bacteraemia: Length of hospital stay (5 vs. 6 vs. 8 days); intensive care admission (7% vs. 20% vs. 23%); pulmonary complications (1% vs. 5% vs. 14%); and 30-day mortality (5% vs. 11% vs. 21%). Compared with patients with no infiltrate and no bacteraemia, the adjusted 30-day mortality rate ratio was 1.9 (95% confidence interval (CI) 0.9-4.1) in patients with an infiltrate without bacteraemia and 4.1 (95% CI 2.0-8.5) in bacteraemia patients. Adjustment for acute disease severity and inflammatory markers weakened these associations.Conclusions: Hospitalization with confirmed pneumococcal LRTI is associated with substantial morbidity and mortality even without positive chest X-ray findings and blood cultures. Still, there is a clinically important outcome gradient from LRTI patients with pneumococcal isolation only to those with detected pulmonary infiltrate or bacteraemia which is partly mediated by higher acute disease severity and inflammation.