Cytologic evaluation is the best way to detect the presence of malignancy in body cavity fluids. Although a positive diagnosis is highly reliable, a negative result does not rule out a malignant cause. Adenocarcinomas, well-differentiated squamous carcinomas, small-cell carcinomas, malignant melanomas, large-cell lymphomas, and acute leukemias are accurately classified when present in effusions. The definitive diagnosis of malignant mesothelioma, small-cell lymphomas, and chronic leukemias, and subclassification of sarcomas and poorly differentiated neoplasms are difficult and may require additional diagnostic techniques. With a few exceptions, the exact causes of benign effusions cannot be determined by cytologic methods.