Pemetrexed is a novel multitargeted antifolate that inhibits > or = 3 enzymes involved in folate metabolism and purine and pyrimidine synthesis. These enzymes are thymidylate synthase, dihydrofolate reductase, and glycinamide ribonucleotide formyltransferase. This agent has broad antitumor activity in phase II trials in a wide variety of solid tumors, and is approved in combination with cisplatin for the therapy of malignant mesothelioma. In a recent phase III trial, pemetrexed demonstrated equivalent efficacy to docetaxel, but with significantly less toxicity, in second-line treatment of non-small-cell lung cancer. The most common and serious toxicities of pemetrexed--myelosuppression and mucositis--have been significantly ameliorated by folic acid and vitamin B12 supplementation. More important, vitamin supplementation has not been shown to adversely affect efficacy in some tumor types. Tumors with codeletion of the methylthioadenosine phosphorylase gene, as a consequence of p16 deletions, may be particularly sensitive to pemetrexed. In this review, the biochemistry and mechanism of action of pemetrexed are discussed.