The injection of sexually immature female rats with 1-methyl-1-nitrosourea results in a rapid induction of premalignant and malignant mammary gland lesions within 35 days of carcinogen administration. This model affords the opportunity for investigators to study the process of mammary carcinogenesis over a very short latency and to investigate early events in this process. We have recently published on various aspects of this system including the histology of the lesions induced, the time frame of their occurrence, and their dependence on ovarian hormones for their maintenance and growth. In this report we present evidence that many aspects of the histopathology of mammary lesions in this model system are similar to those occurring in humans. We also discuss aspects of the human disease, which are not recapitulated in this model.