A statistical model of the natural history of cervical dysplasia, based on a longitudinal study of 557 women, is presented. Other statistical models of carcinogenesis motivated by observations in animal experiments and epidemiological data are reviewed. The observed data obtained by repeated cytological examinations indicate that, once established, a dysplastic lesion rarely spontaneously reverts to normal. The probability of such dysplastic lesions becoming unstable and possibly transiting to another state is estimated. The proposed model is based on the assumptions that these probabilities for a given lesion are constant over time and that the behavior of the lesion in any time interval is independent of its behavior during the preceding observation period. The data obtained in this prospective study are consistent with the projections that are derived, based on the model. In addition, both the observed and predicted results are consistent with the findings of other investigations of cervical dysplasia, carcinoma in situ, and invasive cervical carcinoma.