Here are two cases of two male patients of 57 and 62 years of age, already known as allergic to stinging hymenoptera venom, who after a horsefly bite have presented a serious 3-4 degree-type Mueller classification systemic reaction. The diagnosis has been carried out clinically and after an accurate environmental anamnesis and along with prick tests and RAST, further specific entomological confirm. In literature the so called wasp-mosquito-syndrome has been indicated where hyaluronidase has been referred to as the cross allergen, between the hymenoptera venom and the mosquito saliva, which likely triggers the reaction. We believe that it is also possible to take into consideration a wasp-horsefly-syndrome as well, supposing the increased risk of anaphylactic reactions to Tabanidae bites, relatively frequent in areas with animals and streams, in subjects sensitized to stinging hymenoptera. We also suggest the possibility that in these subjects some systemic reactions are due in fact to Tabanidae bites and not so much for the failure of a possible active ITS of stinging hymenoptera.