Conventional cytological evaluation of serous effusions often yields border line result: apart from positive (malignant) or negative (benign) diagnoses, a relatively large part of the findings are "suspicious for malignancy" (P3). In the present paper the authors have analysed to what extent contributes the determination of cholesterol and CEA levels of ascites and pleural effusion to the diagnostic accuracy of cytologically "suspicious" (P3) cases. In 155 histologically controlled cases, specificity, sensitivity and diagnostic efficiency were assessed on the basis of cytology as well as the determination of CEA and cholesterol levels. Statistical parameters were determined for each method separately and for their combined application. According to the findings, cholesterol and CEA levels over 1.16 mmol/l and 2.5 ng/ml, respectively, indicate malignancy. In 19 out of 29 cytologically "suspicious" cases (66%), which histologically proved to be positive, cholesterol and/or CEA levels were elevated. In all of the 12 non-neoplastic "suspicious" cases the two parameters were under the cutoff values. The application of an easy and inexpensive cholesterol test proves to be a sensitive technique for indicating carcinomatosis and it completes adequately the specific cytological evaluation. If clinical symptoms speak for tumor, but the cytology is negative, the evaluation of CEA level may prove to be useful as far as it can indicate cancer not yet accompanied by carcinomatosis.