This study was designed to investigate the influence of neonatal selenium (an anticarcinogenic agent) exposure on spontaneous liver tumor formation in adult mice. Pregnant CBA mice were administered selenium during the last week of pregnancy and for ten days following parturition. Selenium significantly reduced the incidence of spontaneous hepatomas in adult male progeny, while having no effect on the lower hepatoma incidence in adult females. The data indicate that neonatal selenium alters hepatoma incidence in a sex-dependent manner. This study represents the model of neonatal cancer prevention.