Abstract: Dietary fiber had been previously associated with breast cancer risk but some controversies remain on the role of insoluble fiber type. The aim of the present study was to evaluate the effect of a fiber rich food in an experimental model of mammary carcinogenesis. Animals from study groups received a fiber rich food and were compared with two control groups receiving standard food, two groups received the administration of a common carcinogen and histopathological and blood analysis were conducted. Only four tumors were found on group receiving fiber and the carcinogen against the thirty three tumors found on group receiving only the carcinogen and standard food. Additionally, blood cholesterol and triglycerides were significantly lower in this group (p<0.05). Glycaemia was also lower but without significance. The experimental results had shown a possible protective effect of fiber on mammary carcinomas possible due to carcinogen binding, weight-gain prevention and insulin sensitivity. Further studies should address the effect of the several fiber types on mammary carcinogenesis process.