Pancreatic cystic lesions include inflammatory pseudocysts, benign serous tumors, and mucinous neoplasms, some of which are malignant. Clinical and radiologic indices are often inadequate to discriminate reliably among these possibilities. In an attempt to develop new preoperative diagnostic criteria to assist in decisions regarding therapy, the authors have performed cyst fluid analysis for tumor markers (carcinoembryonic antigen: CEA, CA 125, and CA 19.9), amylase content, amylase isoenzymes, relative viscosity, and cytology on 26 pancreatic cysts. The cases included nine pseudocysts, five serous cystadenomas, 4 mucinous cystic neoplasms, 7 mucinous cystadenocarcinomas, and one mucinous ductal adenocarcinoma with cystic degeneration. Carcinoembryonic antigen levels were high (>367) in all benign and malignant mucinous cysts, but were low (<23) in the pseudocysts and benign serous cystadenomas, an indication that CEA discriminates between mucinous and nonmucinous cysts (p < 0.0001). Values for CA 125 were high in all malignant cysts, low in pseudocysts, and variable in mucinous cystic neoplasms and serous cystadenomas. Levels of Ca 19.9 were nondiscriminatory. Cyst fluid amylase and iipase content were variable but were generally high in pseudocysts and low in cystic tumors. Amylase isoenzyme analysis was useful to differentiate pseudocysts from cystic tumors. Measurement of the relative viscosity in cyst fluid showed high (> serum viscosity) values in 89% of mucinous tumors and low values (< serum) in all pseudocysts and serous cystadenomas (p < 0.01). Cytologic analysis of cyst fluids was of limited value in differentiating pseudocysts from serous cystadenoma, but in seven of eight mucinous tumors provided useful diagnostic information and correctly classified three of five malignant tumors. The authors conclude that cyst fluid analysis can provide a preoperative classification of these diagnostically difficult lesions. The combination of viscosity, CEA, CA 125, and cytology can reliably distinguish malignant cystic tumors and potentially premalignant mucinous cystic neoplasms from pseudocysts and serous cystadenomas. Amylase content with isoenzyme analysis is useful to identify pseudocysts.