The present study indicates that the development of caprine mammary gland warts seems to depend on several factors--namely, nonpigmented skin, adult age, excessive exposure to sunlight, and contact with a yet undefined infective agent. Three types of goat papillomas are described: mammary, cutaneous, other than mammary, and genital. Warts on animals lacking pigmented skin are more frequent in adult animals that live in areas where there is abundant sunlight. Mammary gland papillomas are the most numerous and occur in different stages: ie, goats with mammary gland papillomas that regress, never to recur, goats with papillomas that regress in the winter and reoccur in summer, goats with persistent papillomas of which some are from the group that had previous winter time regression, and goats that have progression of persistent papilloma to carcinoma. Saanen, Saanen crossbreeds, and goats of other breeds that lack pigmented skin and live in sunbelt areas are at high risk for papillomatosis. An infective agent was not defined even after electron microscopic, immunocytochemical, and DNA-DNA hybridization studies. Yet, there is high probability that an infective agent is involved, because mammary gland papillomas usually occur in the susceptible herd 4 to 6 months after an affected goat is introduced.