An increased rate of cell proliferation has long been recognized as an important factor in both human and experimental carcinogenesis, and may be a major risk factor for cancer development in a number of tissues. Limited information exists, however, regarding the relevance of increased cell proliferation and nasal cancer. Examples of toxicological studies utilizing nasal cell proliferation data as an important endpoint are briefly reviewed. Data for one of the most extensively studied chemicals, the weakly genotoxic carcinogen formaldehyde, support the contention that the concentration-response relationship for tumor incidence is a function of formaldehyde-induced target cell proliferation, in addition to other factors including target cell population size. The increasing importance of utilizing cell proliferation data in determining dose-response relationships and in biologically-based risk assessment models is discussed.