Two epileptic patients developed an infectious mononucleosis-like illness which subsequently proved to be a carbamazepine-induced anticonvulsant hypersensitivity syndrome. Patch testing to carbamazepine 3 years later was positive in the one patient tested and negative in normal controls. The second patient died a few weeks after the illness, secondary to long-standing cardiac disease without having undergone patch testing. A skin biopsy was, however, consistent with an immune complex mediated drug reaction. Patch testing for systemically administered drugs is generally believed to be of little value in diagnosing drug allergies. However, we reinforce a previous suggestion that this investigation may be helpful in some cases of anticonvulsant hypersensitivity syndrome caused by carbamazepine. The pathogenic role of type 3 and 4 hypersensitivity is also discussed.