To the Editor.—Piritrexim (2,4-diamino-6[2,5-dimeth-oxybenzyl]-5-methylpyrido-[2,3-D]pyrimidine) (PTX) (BW-301U) is a novel lipid-soluble antifolate that has looked promising in clinical trials.1It is as active as methotrexate as an inhibitor of dihydrofolate reductase. However, in contrast to methotrexate, it crosses membranes by passive diffusion and may be able to overcome cellular resistance to methotrexate due to folate transport mutations. Piritrexim, unlike methotrexate, is not polyglutamated and is potentially less hepatotoxic, since it is not stored in the liver as are methotrexate polyglutamates.2Since methotrexate is effective in the treatment of cutaneous T-cell lymphoma,3a trial of piritrexim in mycosis fungoides seemed reasonable. Unlike methotrexate, piritrexim has little anticancer effect when given as a single dose or over 24 hours. Therefore, the low-dose long-term administration schedule effective against neoplasms was used in this study.Nine patients (six women and three men; age range, 41 to 71 years) enter