Cytologic specimens of 105 pericardial fluids collected from 95 cases during a seven-year period were reviewed. Clinical reports and descriptions of the histologic antemortem and postmortem specimens were correlated with the cytologic diagnoses, and the interobserver variation was estimated. Of the collected material, 48.4% was from patients suspected of having nonmalignant disorders, 40.0% was from patients with previously diagnosed carcinomas and 11.6% was from cases in which the etiology was unknown at the time of pericardiocentesis. Cytologic examination of the pericardial fluids revealed tumor cells in a sample from one patient suspected of having a heart disorder and in a sample from another patient with an obscure disease. Of the pericardial fluids from the cancer patients, 66.7% contained malignant cells; the most frequent primary site in these cases was the lung. Correlated with the histologic diagnosis, the specificity of cytology was 100%. The results prove that, in experienced hands, pericardial cytology is a valuable diagnostic tool.