generalized maculopapular exanthem beginning on the hands, one day after the administration of chlorazepate. The pach test with chlorazepate was positive. The prick test and intradermal test with chlorazepate were negative; the lymphocyte transformation test showed proliferative lymphocyte reactivity to chlorazepate in the culture supernatants of peripheral blood mononuclear cells, coincubated with human liver microsomes, whereas in the culture cells without the addition of microsomes no significant proliferation was observed. We expose a case of probable immediate hypersensitivity to the chlorazepate dipotassium. Although the speed of the appearance of the symptomatology and the clinical characteristics of the episode would seem to point towards an IgE-mediated reaction, the negativity of the cutaneous tests prevents us establishing the pathogenic mechanism with total certainty. It seems clear that it is not a delayed hypersensibility reaction, because of the quickness of the symptomatology and negative patch test result. Our patient presented the same symptomatology after the administration of diazepam, although he tolerated the provocation with bentazepam. It is not surprising the existence of crossreactivity between chlorazepate and diazepam, because of the similar chemical structure of both molecules (Fig. 1), composed by a benzodiazepinic ring with another benzene ring adhered to 5 carbon. In addition both molecules are metabolized in the liver giving rise to the same intermediate active metabolite, the nordiazepam, which probably also accounts for the immunological adverse effects (3, 6). On the contrary, bentazepam belongs to a different drug subgroup called tienodiazepines, which differ in their chemical structure, by the substitution of the benzene ring by a tiofene ring adhered to the diazepinic ring, maintaining the other benzene ring in the 5 carbon extreme. Brotizolam, clotiazepam and etizolam also belong to this group. We thank S. Espinoza and R. Diaz for their collaboration. *Service of Allergology Clinical Hospital of Valencia Avda. Blasco Ibanez, 17 46010 Valencia Espana E-mails: raceraz@hotmail.com; pelaez_ant@gva.es