Seventy exudates from 45 patients with clinically suspected malignancy were examined by cytology, cytophotometric measurement of DNA, short term cell culture, scanning and transmission electron microscopy. In 7 patients (21 fluids), the presence of malignant disease was verified. Malignant and benign cases were correctly diagnosed by combination of cytology and DNA analyses. An abnormal DNA profile defined by > 10% cells with > 2c DNA or single cells with > 8c DNA was only seen in malignant exudates. Short term cell culture with scanning electron microscopy could distinguish between lymphoid cells, histiocytes, fibroblasts, mesothelial cells and cancer cells. Only cancer cells and prominent microvilli on their surface. A future larger series will explore whether a combination of cytology and cytophotometric DNA estimation alone will improve the diagnostic accuracy to the same substantial degree as this pilot study would suggest.