The retinoblastoma gene (RB) is the prototypic tumor suppressor. Studies to date have demonstrated cancer suppression with tumor cells reconstituted with RB ex vivo and implanted into immunodeficient mice, as well as with germline transmission of a human RB transgene into tumor–prone Rb^+/− mice. To mimic the therapy of cancer more closely, spontaneous pituitary melanotroph tumors arising in immunocompetent Rb^+/− mice were treated with a recombinant adenovirus carrying RB cDNA. Intratumoral RB gene transfer decreased tumor cell proliferation, reestablished innervation by growth–regulatory dopaminergic neurons, inhibited the growth of tumors, and prolonged the life spans of treated animals.