AIMS: To determine the level of proliferative activity in 39 nasal polyps with clear cut distinct clinical behaviour patterns. METHODS: The 39 nasal polyps included 11 polyps labelled as "single" and taken from the lateral nasal wall and the middle turbinate; 12 polyps labelled as "massive" and relating to diffuse polyposis involving the entire nasal cavity; six polyps labelled as "ASA" and relating to nasal polyps from patients with acetylsalicylic acid intolerance and asthma; and 10 polyps from cystic fibrosis related polyposis. Cell proliferation was determined by two independent methods: first, the computer assisted microscope analysis of isolated Feulgen stained nuclei for the measurement of the percentage of cells in the S phase of the cell cycle; and second, the immunohistochemical evaluation of a proliferation associated protein by means of the MIB 1 monoclonal antibody. RESULTS: The cystic fibrosis related polyposis exhibited the highest proliferative activity of all the clinically identified nasal polyp groups. Acute inflammatory nasal polyps exhibited a higher cell proliferation than chronic ones. The results also show that while the immunohistochemical determination of cell proliferation by means of the MIB 1 monoclonal antibody is a valuable tool in determining cell proliferation in nasal polyps, the cytometrical image analysis of Feulgen stained nuclei is not useful for this purpose. CONCLUSION: Cell proliferation activity identifies cystic fibrosis as being distinct from the other nasal polyp groups.