Both a low pleural fluid glucose concentration and pleural fluid acidosis are markers of severe pleural inflammation, but the relationship between these phenomena has not been defined clearly. Therefore, we measured simultaneous pleural fluid glucose concentrations and pH in 25 consecutive parapneumonic pleural fluids. Seventeen effusions had a glucose concentration greater than 60 mg/dl (group 1, 126 +/- 7 mg/dl, mean +/- SEM), while eight had a pleural fluid glucose less than 60 mg/dl (group 2, 15 +/- 3 mg/dl, P less than .01). Pleural fluid pH was 7.35 +/- 0.03 in group 1 compared with 6.83 +/- 0.09 in group 2 (P less than .01). A significant correlation between pleural fluid glucose and pH was found (r = .81, P less than .01). Low-glucose, low-pH effusions were complicated (either loculated or empyemas). Uncomplicated effusions had glucose concentrations greater than 60 mg/dl and a pleural fluid pH greater than 7.30. The concomitant occurrence of low pleural fluid glucose and pH suggests that the mechanisms leading to these phenomena are interrelated.