Serum IgD levels were determined by radial immunodiffusion among 349 patients with various dermatoses and 40 normal blood donnors. An increase of serum concentrations was found in atopic patients 62,5 p. 100 of which had levels over 50 mg/1 compared to 20 p. 100 in controls. No correlation was found between IgD and IgE levels in these patients. An increase (but statistically not significant) was also found in chronic urticaria whereas levels were normal in acute urticaria. No difference was found in contact dermatitis nor in psoriasis. High levels occurred in most of patients with primo-secondary syphilis and in acne pustulosa, whereas low IgD levels were found in most patients with malignant proliferative diseases (mycosis fongoide, malignant melanoma, carcinoma). However, the number of patients tested in these groups is too small to allow definitive conclusions. In our experience, determination of serum IgD values is not very useful for diagnosis in dermatologic patients.