A 54-year old male developed unexpectedy a severe penicillin allergy after 5 standard erysipelas treatments by intravenous benzylpenicillin and/or oral phenoxymethylpenicillin without any clinical symptoms of allergy during 16 years. During the 6th erysipelas treatment by intravenous benzylpenicillin, the patient experienced an anaphylactic reaction within 30 minutes.