Trimetrexate is a promising new lipophilic antifolate with antitumor and antimicrobial activity, which is currently undergoing clinical trials. It differs from methotrexate in its transport and intracellular retention, and may be useful against tumors resistant to methotrexate because of impaired transport or deficient polyglutamylation. In preclinical studies, it has demonstrated a broader spectrum of antitumor activity than methotrexate, and appears to have synergistic antitumor activity when combined with a number of other agents. In clinical trials, the primary toxicity has been hematologic, with rapid recovery and no evidence of cumulative dose effect. Nonhematologic toxicity has been sporadic. Preliminary results of phase II trials indicate activity against breast, non-small-cell lung, and head and neck tumors. In addition, trimetrexate with simultaneous leucovorin rescue appears to be a promising new treatment for pneumocystis pneumonia in AIDS patients, with minimal toxic side effects.