Interferons, a group of naturally occurring proteins, possess potent immunological effects. The availability of large amounts of purified interferon through recombinant DNA technology has led to the testing of these agents against a number of tumor types. Interferon-alpha has been the most widely studied. The most encouraging results have been observed in the treatment of melanoma among nonhematological neoplasms, and in the locoregional treatment of other tumor types. It is hoped that further research into the use of the interferons, both singly and in combination with chemotherapy and other immunological agents, will lead to a better elucidation of their therapeutic role against cancer. In this article, the use of interferons in the treatment of solid tumors is reviewed.