4-Ipomeanol (IPO) is the first agent to undergo preclinical development at the National Cancer Institute (NCI) based principally on a specific biochemical-biological rationale for clinical investigation as an antineoplastic agent targeted against lung cancer. This disease-specific development of IPO was initially stimulated by observations that the compound was activated by metabolism, preferentially within the mammalian lung, specifically within bronchiolar Clara cells, and that its predominant toxicity was to the lung in most species. IPO is inactive or only minimally active against most conventional antitumor test systems. However, some human lung cancer cell lines, as well as a variety of fresh human lung tumor biopsy specimens, have been shown to be capable of mediating the in situ biotransformation of IPO to a potentially cytotoxic intermediate. In this report, the biochemistry, metabolism, preclinical pharmacology, and toxicology of IPO are reviewed and the clinical development plans for this unique and challenging new agent are presented.