This study aimed to evaluate the relations between the levels of CRP, leukocyte count and ESR on admission and the severity of pneumonia according to the criteria of Turkish Thoracic Society (TTS) and British Thoracic Society (BTS) CAP guidelines. This study included the adult patients with CAP admitted to our clinic between the years 2003-2005. The history, physical findings, hemogram, ESR, the levels of CRP and the results of other laboratory investigations were obtained from the medical records. The patients were grouped according to BTS and TTS guidelines. The mean age was 47.2 years; 70 patients (75.3%) were male and 23 patients (24.7%) were female. The severity of pneumonia according to BTS criteria was correlated with the levels of CRP and leukocyte count (p= 0.037, p= 0.01, respectively). The severity of pneumonia according to TTS criteria was correlated with the levels of CRP, leukocyte count and ESR (p= 0.000, p= 0.014, p= 0.015, respectively). Among TTS pneumonia groups, there were statistically significant differences between groups 1 and 3; groups 1 and 4; groups 2 and 3 (p= 0.006, p= 0.041, p= 0.05, respectively) for mean CRP levels. The mean levels of CRP (103.2 +/- 76.4 mg/L), leukocyte count (19.8 +/- 9.5 x 10(3)/microL) and ESR (57.2 +/- 26.8 mm/hour) were statistically significantly higher in inpatients than the mean levels of CRP (53.2 +/- 52.8 mg/dL), leukocyte count (14.6 +/- 5.4 x 10(3)/microL) and ESR (43.1 +/- 25.9 mm/hour) in outpatients (p= 0.000, p= 0.001, p= 0.012, respectively) according to TTS. It is considered that CRP, a powerful marker of inflammation, is related with severity of pneumonia and a high level of CRP may be useful to make a decision about hospitalisation.