Carbamazepine (C) can cause a characteristic hypersensitivity reaction (CHS¿. This multisystem reaction typically presents as fever, mucocutaneous eruption and lymphadenopathy. The syndrome usually develops between 1 week and 3 months after starting therapy, with involvement of the liver, lung, kidney and inappropriate secretion of ADH. The incidence is less than 0.001% in those treated with C and it is diagnosed clinically. With onset of CHS, the drug must be stopped and if there is no improvement, cortico-steroids should be started. When the diagnosis is in doubt, the patch test, lymphocyte transformation test, macrophage migration inhibitor factor, and other tests can be helpful. The pathogenesis is not known. Similar syndromes have been described with phenytoin and phenobarbital. There is clinical and in-vitro evidence of cross reactions between C and phenytoin. It is not known whether the CHS syndrome should be considered a premalignant state, with increased risk for the development of malignant lymphoma.