A 36-year-old woman and a 16-year-old boy, both suffering from mycosis fungoides, developed urticaria and an anaphylactoid reaction after topical whole body application of nitrogen mustard. Prick tests with nitrogen mustard solution produced a weal and flare response. Both patients had previously been treated intermittently with total body application of nitrogen mustard for 2 1/2 years and 1 year respectively without complications.