The aim of this study was to determine the relationship between C-reactive protein (CRP) levels and factors known to predict outcome in stable chronic obstructive pulmonary disease (COPD) patients. The following were studied in 130 stable COPD patients: spirometry, lung volume, arterial oxygen tension (P(a,O2)), dyspnoea, 6-min walk distance (6MWD), body mass index, fat-free mass index, BODE (body mass index, obstruction, dyspnoea and exercise capacity), health-related quality of life, smoking status, the presence of cardiovascular risk factors or disease, corticosteroid use and number of exacerbations in the previous year. CRP levels were measured in these patients and in 65 controls. Using univariate and multivariate analyses, any possible association with the predictors of outcomes was evaluated. CRP levels were higher in COPD patients than in controls (4.1 versus 1.8 mg.L(-1), respectively). Correlation was found with the following variables: forced expiratory volume in one second (FEV1; -0.23), FEV1 % (-0.20), forced vital capacity (FVC; -0.24), FVC % (-0.24), Global Initiative for Chronic Obstructive Lung Disease stage (0.17), BODE (0.17), inspiratory capacity/total lung capacity (-0.20), P(a,O2) (-0.40) and 6MWD (-0.30). Using multivariate analysis, P(a,O2) and 6MWD manifested the strongest negative association with CRP levels. C-reactive protein levels in stable chronic obstructive pulmonary disease patients are best correlated with arterial oxygen tension and 6-min walk distance. This should be considered when C-reactive protein levels are measured in stable chronic obstructive pulmonary disease patients.