Objective: To investigate the value of C-reactive protein (CRP) as a marker of chronic obstructive pulmonary disease (COPD) exacerbations or specifically bacterial exacerbations and to evaluate a correlation between raised CRP levels and other markers of inflammation in patients with an acute exacerbation (AECOPD). Subjects and Methods: The medical records of patients with AECOPD were retrospectively analyzed. They were categorized according to the nature of sputum as mucoid or purulent and to the findings on chest radiographs as with pneumonia (PCOPD) or without pneumonia. Stable COPD (SCOPD) patients and a group of asymptomatic nonsmokers were also included in the study. Results: All COPD patients (SCOPD: 30; AECOPD: 51; PCOPD: 32) and control subjects (30) were male. The mean CRP levels and WBC counts of the groups were PCOPD: 108.1 +/- 61.8 mg/l and 13.7 +/- 6.8 x 10(9)/l; AECOPD: 36.8 +/- 43.9 mg/l and 11.4 +/- 4.8 x 10(9)/l; SCOPD: 3.9 +/- 1.4 mg/l and 7.9 +/- 1.9 x 10(9)/l; control: 2.1 +/- 0.9 mg/l and 7.7 +/- 1.1 x 10(9)/l. The mean CRP level of AECOPD was statistically different from those of PCOPD and SCOPD (p = 0.0001, p = 0.002, respectively). The sensitivity and specificity of CRP to determine an acute exacerbation were 72.5 and 100%, respectively. Among the patients with AECOPD, 25 had purulent sputum and a mean CRP level of 46.4 +/- 48.6 mg/l, which is significantly higher than the CRP level (28.0 +/- 44.5 mg/l) of the 18 patients with mucoid expectoration (p = 0.015). Among the mucoid-expectorating subgroup, the patients with leukocytosis had significantly higher CRP levels than the patients without leukocytosis (p = 0.034). Conclusion: A high serum CRP value may indicate an infectious exacerbation in COPD patients and it correlates with sputum purulence and increased serum WBC counts. Copyright (C) 2008 S. Karger AG, Basel.