We show that 9-cis-retinoic acid (9cRA) is a potent inhibitor of mammary carcinogenesis induced by N-nitroso-N-methylurea in Sprague-Dawley rats. Rats were first treated with a single dose of N-nitroso-N-methylurea (50 mg/kg body weight) and then fed non-toxic levels of 9cRA (120 or 60 mg/kg of diet). 9cRA was highly effective in reducing tumor incidence, average number of tumors per rat, and average tumor burden, as well as extending tumor latency. The combination of 9cRA with low levels of tamoxifen (TAM; fed at either 1.0 or 0.5 mg/kg of diet) was particularly effective; addition of 9cRA to a TAM regimen doubled the number of animals that were tumor-free at autopsy and significantly diminished tumor number and tumor burden. For suppression of carcinogenesis in vivo, 9cRA was much more potent than all-trans-retinoic acid, both as a single agent or in combination with TAM, although both retinoids had equivalent inhibitory effects on DNA synthesis in cultured human breast cancer cell lines. Both 9cRA and all-trans-retinoic acid induce the expression of the adhesion molecule, E-cadherin, in the SK-BR-3 cell line. We suggest that clinical evaluation of the combination of 9cRA and TAM, either for chemoprevention or for adjuvant therapy, should be considered.