The aim of this study was to elucidate the role of the C11-OH substituent of the corticosteroid molecule in relation to contact allergy to corticosteroids. This group, which is essential for glucocorticoid activity, is not present in other steroid molecules, such as deoxycortisol, progestagens, testosterone, and estrogen. Skin tests with these substances in hydrocortisone-allergic patients revealed a division between patients for whom the C11-OH group was essential to induce the allergic reaction and patients for whom this group did not seem to be essential. Some of the patients of the latter group also reacted to 17-alpha-OH-progesterone. This may indicate cross-sensitivity between hydrocortisone and 17-alpha-OH-progesterone, which might play an etiological role in autoimmune progesterone dermatitis.