Chromosome analysis of effusions has been recently regarded as useful means in the diagnosis of cancer. Cytological and cytogenetic findings of 19 pleural effusions from patients with benign and malignant diseases are compared. Conventional cytology does not always give correct positive results because of the high percentage of false negatives, whereas cytogenetic analysis reveals a considerable spread of the chromosome number in neoplastic fluid, with structural chromosome changes, marker chromosomes and minute fragments. Absence of mitosis does not exclude the malignant etiology of the effusion when the patient had been previously treated with antineoplastic drugs. Benign diseases were never falsely classified as malignant by cytogenetic analysis.