Overexpression of a transforming growth factor alpha (TGF-alpha) transgene induced the development of liver tumors in 69 of 93 (74%) adult male mice. To identify factors associated with oncogenesis, liver tumors from transgenic animals were characterized at the molecular level. TGF-alpha RNA transcripts were elevated in 17 of 25 (68%) liver tumors, relative to adjacent nontumorous tissue. Expression of the endogenous c-myc and insulin-like growth factor II genes was enhanced in 7 of 19 (37%) and 12 of 16 (75%) tumors, respectively. In contrast, epidermal growth factor receptor RNA levels were unchanged or reduced in all liver tumors, and mutations were not detected in either the Ha-ras or Ki-ras genes. The occurrence of liver tumors in castrated TGF-alpha transgenic mice was reduced about 7-fold, while in ovariectomized transgenic animals the incidence was increased about 6-fold. The progeny of a cross between CD1-derived TGF-alpha transgenic (MT42) and C57BL/6 mice exhibited no reduction in tumor burden (83%); however, the incidence of tumor formation in MT42 x FVB/N offspring was substantially lower (19%). We conclude that in these transgenic mice TGF-alpha promotes tumor formation and appears to play a major role in tumor progression. Moreover, other factors that may collaborate in TGF-alpha-induced hepatocarcinogenesis include c-myc, insulin-like growth factor II, sex hormones, and the genetic background upon which the transgene operates.