We retrospectively analyzed the severity of community-acquired pneumonia in 293 patients. Based on the Japanese Respiratory Society (JRS) risk stratification guidelines (A-DROP), patients were classified as follows: mild, 74 (25%); moderate, 140 (48%); severe, 53 (18%); and extremely severe, 26 (9%). The mortality of each category was classified as follows: mild, 0 (0%); moderate, 4 (3%); severe, 8 (15%); and extremely severe, 8 (31%). Based on the Pneumonia Severity Index (PSI) score used in the Infectious Disease Society of America/American Thoracic Society (IDSA/ATS) consensus guidelines, patients were classified as follows: I, 36 (12%); II, 44 (15%); III, 61 (21%); IV, 92 (31%); and V, 60 (20%). The mortality of each category was classified as follows: I-III, 0 (0%); IV, 5 (25%); or V, 15 (75%). Comparisons made between patients who died within 14 days and those who survived, showed that leukocytes and CRP values were higher among the fatality group. Moreover, Alb/BUN correlated with the PSI score (r = - 0.62). We propose Alb/BUN as a new index for prognostic factor of community-acquired pneumonia.