Pleural fluid characteristics of 26 patients diagnosed with malignant mesothelioma over an 18-year period were reviewed and compared with those of patients with effusions due to other malignancies. Survival from time of initial thoracentesis was directly correlated with pleural pH and decreased pleural fluid/serum glucose ratios but was not related to protein or LDH concentration. Decreased pleural fluid pH was statistically related to a decreased pleural fluid/serum glucose ratio. Patients with low pH (less than 7.30) pleural effusions had statistically decreased mean survival and decreased mean pleural fluid/serum glucose ratios compared with patients with normal pH (greater than or equal to 7.30). Patients with malignant mesothelioma pleural effusions had statistically decreased mean pleural fluid pH compared with patients with other malignant pleural effusions. Decreased levels of pleural fluid pH and pleural fluid/serum glucose ratios have negative prognostic significance and probably correlate with the degree of tumor bulk affecting the pleural surface.