Methotrexate (MTX) in high doses (3 to 7.5 g/m2) with leucovorin rescue (HDMTX-LCV) can be delivered on a weekly basis in a setting of proper pharmacologic monitoring. Myelosuppression occurs in 28 per cent of the patients and in 8 per cent of the courses and usually results from delayed MTX excretion secondary to mild reversible nephrotoxicity. The incidence of tumor regression was 50 per cent in head and neck cancer; 59 per cent in non-Hodgkin's lymphoma; 40 per cent in small cell lung cancer; 24 to 50 per cent in breast cancer and 50 per cent in osteogenic carcinoma, for an over-all response rate of 39 per cent (70 of 178) in patients with disseminated cancer. HDMTX-LCV is not recommended for the conventional treatment of metastatic cancer because of the potential for toxicity and the fact that the response rates cited are probably not superior to those which can be achieved by conventional doses of MTX. However, the relative lack of myelosuppression and mucositis, when compared to conventional unrescued MTS, and the achievement of therapeutic concentrations of MTX in the central nervous system with the HDMTX-LCV program have led to its incorporation into clinical trials of combination chemotherapy.