Fifty nine effusions (6 from patients with benign diseases and 53 from cancer patients) were examined cytologically and cytogenetically. Only diploid cells were observed in the benign effusions. The majority of malignant effusions had chromosomal changes, but in three cases where cytologic examination had revealed the presence of malignant cells, only diploid metaphases were found. No false positive diagnoses were made either by the cytologic or by the cytogenetic technique. The concomitant use of cytologic and cytogenetic examinations has substantially improved the correctness of the diagnosis. If by cytologic examination 43 of the 53 effusions from cancer patients were diagnosed as positive, 4 as suspicious and 6 as negative, the combined results of the cytologic and cytogenetic examination can be summarized in 49 positive, 1 suspicious and 3 negative cases. The implications of these data for the medical practice are briefly discussed.