The cancer protective effects of flaxseed suggested by our previous short-term study were tested in a long-term tumorigenesis experiment. Feeding rats 5% flaxseed flour supplemented in a high-fat diet at the promotional stage of tumorigenesis, i.e., after 7,12-dimethylbenz[a]anthracene administration, significantly reduced by 66.7% the size of the tumours that occurred. Although flaxseed feeding at the initiation stage also tended to reduce the number of tumors per tumor-bearing animal, significant differences were seen only between the group fed flaxseed throughout the experiment and the promotional group. Therefore the effect of flaxseed on mammary tumorigenesis is not consistent. Although it was speculated that the effect may be related to the lignans enterolactone and enterodiol produced in such large quantities on the ingestion of flaxseed, further studies are required to clarify the role of lignans and other flaxseed constituents in mammary tumorigenesis.