The retrospective flow cytometric analysis of 54 colorectal adenomatous polyps resected endoscopically from 43 patients was performed. The aim of the study, with the use of flow cytometry was to determine, DNA ploidy and proliferative activity of epithelial cells in a series of adenomas and to compare the results with classical histopathological criteria. Overall 30 tubular and 19 tubulo-villous/villous adenomas with different grade of dysplasia as well as 5 adenomatous polyps with carcinoma in situ (CIS) were examined. DNA aneuploidy was found in 4 of 54 polyps (7.4%), all with histological features of CIS. Proliferation rate increased significantly with the degree of dysplasia (9.0 +/- 3.1%; 16.3 +/- 4.3%; 22.1 +/- 4.1% in adenomas with mild, moderate and severe dysplasia respectively; p < 0.05). No significant differences in cell proliferative activity were found between groups of adenomas as compared to histological type and size. The results show that cell proliferative activity in colorectal adenomatous polyps depends strongly on the grade of dysplasia and does not show such a direct association with respect to histological type or size of adenomas. It may be concluded as well that DNA aneuploidy precedes histological changes of invasive carcinoma in colorectal adenomas and therefore flow cytometric DNA analysis may be of great value in defining more accurately the biological stage of neoplasmatic process in individual cases.