Background: The effect of chemopreventive agents on cancer multiplicity is of primary interest in animal studies. The nature of data collected from chemoprevention studies may be analyzed by a longitudinal analysis of repeatedly measured cancer multiplicity data. Methods: We determined the number of mammary cancers over the entire follow-up period for varying doses of two chemopreventive agents. Longitudinal analyses were performed to model the number of cancers over different time intervals. Results: There was a significant increase in the number of cancers between six to seven weeks post-carcinogen administration in the control group. Varying patterns of cancer development were observed at different doses of chemopreventive agents including a delay in onset of tumor growth compared to the control group. Conclusion: Longitudinal data analysis complements traditional analyses by providing detailed information regarding the effect of chemopreventive agents on the pattern of tumor development throughout the follow-up period. Importantly, some chemopreventive agents may delay time to appearance of mammary cancers without causing a significant difference in cancer multiplicity.