Since the earliest report1 of the use of antimetabolites in the treatment of psoriasis, the secondary toxic manifestations have precluded their general acceptance. While there have been favorable comments on the use of aminopterin (4-aminopteroyl glutamic acid) in a few reports,2-4 attention has also been drawn to the lack of responsiveness to low dose levels5 and to the toxic effects of this drug when used in sufficiently high dosages.6 Gastrointestinal toxicity was noted in 2 out of 3 patients first treated with aminopterin on this service. Hence, the application of other antimetabolites to the therapy of refractory psoriasis was undertaken. Mercaptopurine (6-mercaptopurine, Purinethol) is a synthetic analogue of adenine and of the purine base hypoxanthine. It is reported to interfere with nucleic acid biosynthesis.7 There is one report of apparent failure of psoriasis to respond to a very limited course of therapy with the drug.