During puberty, human and rodent mammary glands are highly sensitive to cancer-causing agents. A high in animal fat diet (HFD) initiated during puberty in BALB/c mice reduced the latency of carcinogen-induced mammary tumors, without development of obesity or metabolic syndrome We sought to identify a signature for early occurring (HFD-E) tumors and to test whether HFD exposure restricted to puberty was sufficient to promote mammary tumorigenesis. Pubertal BALB/c mice were fed a non-isocaloric low fat diet (LFD; 12% kcal fat) or HFD (60% kcal fat), and subjected to DMBA-induced tumorigenesis. Some mice initiated on either diet at 3 weeks of age were switched from HFD to LFD or LFD to HFD at 9 weeks of age, after 6 weeks on the first diet. Pathway-targeted PCR-array analyses revealed that HFD-E tumors uniquely upregulated genes associated with genotoxic stress and stem and/or progenitor cell characteristics. Microarray analyses confirmed a unique gene expression signature for HFD-E tumors, and confirmed upregulation of pathways involving b-catenin. Switching animals from HFD to LFD did not reverse HFD-enhanced tumorigenesis, demonstrating a specific peri-pubertal window of susceptibility to HFD effects. Importantly, short-term exposure to HFD in the peri-pubertal period was sufficient to increase mammary tumorigenesis similarly to lifelong HFD exposure. “HFD to LFD” switch mice also had increased abnormal mammary hyperplasias and enhanced cellular proliferation. Upon switch to LFD, mice did not sustain increased angiogenesis, but showed increased levels of tumor-associated macrophages similarly to animals on lifelong HFD. Adenosquamos tumors predominated among the HFD-E tumors. DMBA mutagenesis is virtually random in its targeting of genes in tumorigenesis; however, the unique gene expression profile of HFD-E tumors, highlights the overarching influence of HFD. This gene expression profile suggested similarity between the HFD-E tumors and human breast tumors displaying stem and/or progenitor cell characteristics, such as basal-like breast cancer.