To estimate the minimal latent period for malignant mesothelioma of occupational origin, we reviewed 21 articles that documented latent periods for malignant mesothelioma, totaling 1690 cases. Of these, 1105 (65%) cases fulfilled the strict histologic and exposure criteria and were included in this analysis. Of these mesotheliomas, 99% had a latent period more than 15 years; 96% had a latent period of at least 20 years; and the observed probability of an occupational exposure within a decade of the first exposure was zero. The estimated median latent period was at least 32 years after the initial exposure. These epidemiologic conclusions are important in determining the source of a specific case of the disease, understanding the natural history of the disease, determining liability, and in aiding in the diagnosis of malignant mesotheliomas of occupational origin.