Immunoglobulin E was determined in 13 patients with cutaneous lymphoma and in 30 patients with parapsoriasis en plaques. 11 out of the patients with cutaneous lymphomas showed mycosis fungoides. In 8 patients with parapsoriasis en plaques serum IgE was elevated. These patients revealed signs of the premalignant form of parapsoriasis en plaques. In mycosis fungoides elevated IgE is common, but normal levels are found, too.