Flaxseed is a dietary source of possible chemopreventive compounds such as lignans and α-linolenic acid (ALA). To study the effects of a flaxseed mixture on adenoma formation in multiple intestinal neoplasia mice, the mice were fed a diet containing 2·7 % flaxseed, 4·5 % fibre and 3·7 % ALA. To elucidate the effect of oils of the mixture we also composed a diet without flaxseed but with the same oil composition. The median number of adenomas in the small intestine was fifty-four for the control group, and thirty-seven (P=0·023) and forty-two (P=0·095) for flaxseed and oil groups, respectively. Compared with controls (1·2 mm), the adenoma size was smaller in the flaxseed (0·9 mm; P=0·002) and oil (1·0 mm; P=0·012) groups. Both diets changed the proportions of n-3 and n-6 fatty acids in the colonic mucosa. Membrane β-catenin and protein kinase C (PKC)-ζ levels were reduced in the adenoma v. mucosa (P<0·05), and an inverse association was found between the membrane PKC-ζ in the mucosa and the adenoma number (r −0·460, P=0·008, n 32). Only the flaxseed diet increased lignan levels in the caecum (P=0·002) and in plasma (P=0·002) but they were not associated with tumour formation. The results suggest that the preventive effect of flaxseed on colon carcinogenesis may be due to the oil part of flaxseed, and the loss of β-catenin and PKC-ζ from the membranes of the mucosal tissue may play a permissive role in intestinal tumour development.