Community-acquired pneumonia (CAP) has a high incidence and involves an important consumption of healthcare resources. The present authors analysed the influence of comorbidity, initial severity and complications upon the direct costs associated with hospitalised CAP patients. Direct hospitalisation costs (room cost, treatment, laboratory and diagnostic tests) were assessed in a prospective, observational study of 271 patients admitted to a hospital ward due to CAP. The mean+/-SD patient age was 70+/-15 yrs. The mortality rate was 11.1%. Complications were found in 72.3% and comorbidities in 74.9%. The median (interquartile range) total cost was 1,683 euros (1,291-2,471 euros) and the component costs were: room cost 1,286 euros (857-1,714 euros); laboratory tests 212 euros (171-272 euros); treatment 187 euros (114-304 euros); and diagnostic procedures 58 euros (29-122 euros). Complications and higher Pneumonia Severity Index increased the costs, but age and comorbidity did not. A logistic regression analysis to predict high cost (>1,683 euros) showed that infectious (odds ratio 6.8, 95% confidence interval 1.3-36), digestive (5.9 (1.5-22.8)), pulmonary (2.6 (1.4-4.7)) and other complications (3.9 (1.8-8.4)) were independent risk factors, as were previous hospitalisation (2.3 (1.2-4.3)) and hypoalbuminaemia (2 (1.1-3.6)). Complications, hypoalbuminaemia and previous hospitalisation were the main determinants of high direct costs of hospitalisation due to community-acquired pneumonia. Neither age nor comorbidities were independently associated with cost.