Methotrexate is a drug used solely or in combination with other anti-neoplastic drugs for the treatment of certain neoplastic diseases, severe psoriasis, and adult rheumatoid arthritis. Methotrexate has the potential for serious toxicity. Patients undergoing methotrexate therapy should be closely monitored so that toxic effects are detected promptly. Relatively low doses of methotrexate (approximately 7.5 - 25 mg/week) have been used in the treatment of nonmalignant diseases such as severe psoriasis, asthma, rheumatoid arthritis, sarcoidosis, and transplant therapy. Intermediate to high doses of methotrexate (approximately 35 mg/m2 – 12 g/m2) with leucovorin (citrovorum-factor) rescue have been used with favorable results in the treatment of osteogenic sarcoma, leukemia, non-Hodgkin’s lymphoma, and lung and breast cancer. The efficacy of methotrexate in the treatment of other tumors such as prostatic cancer is being investigated.