Methotrexate is now used widely for the treatment of acute leukemia, non-Hodgkin's lymphoma, osteogenic sarcoma, choriocarcinoma, breast carcinoma, pulmonary and epidermoid carcinoma, and intrathecal chemotherapy. It is also useful in bone marrow transplantation, severe psoriasis, rheumatoid arthritis, dermatomyositis, Wegener's granulomatosis and sarcoidosis. The recent dramatic intensification of methotrexate therapy can be attributed in part to advances in our understanding of the clinical pharmacology of the folate antagonists, as well as to the combination of positive results and their effective dissemination to medical oncologists. The review summarizes the pharmacologic findings and illustrates how they are currently being applied to the treatment of malignant disease.