A 65-year-old woman developed haemorrhagic diathesis due to a profound thrombocytopenia (thrombocyte count: 1 x 10(9)/l) within one week after a 10-day course of norfloxacin (2 x 400 mg/day), prescribed for cystitis. On account of increased megakaryopoiesis in the bone marrow and absence of other causes of thrombocytopenia norfloxacin-induced immune thrombocytopenia was diagnosed. No norfloxacin-dependent antibodies against platelets were detected. Treatment with prednisone (1.5 mg/kg/day) resulted in the normalization of the platelet count within 5 days.