We studied an acute generalized exanthematous pustulosis (AGEP) due to sulfamethoxazol in a 48-year-old woman with unusual findings in allergy testing. The histological picture provided evidence for a pustular drug eruption and leukocytoclastic vasculitis. Skin testing with sulfamethoxazol was negative for immediate-type reaction (scratch test) and delayed-type reaction (epicutaneous testing). A lymphocyte transformation test (LTT) showed a significant lymphocyte stimulation (stimulation index 5.04/2.61) toward sulfamethoxazol (200/100 mg/ml) by measuring the rate of built-in tritium-thymidine in the DNS of the patients lymphocytes, implicating a drug-specific hypersensibility of lymphocytes; we could be dealing with a combined type III and IV reaction by Coombs and Gell in this case. LTT may play a possible role in the determination of drug allergy in AGEP despite negative skin testing.