Medical leeches (Hirudo medicinalis) are an important therapeutic option in the treatment of venous congestion of flaps. In a case report an infection with Aeromonas hydrophila of a free microvascular osteo-(myo)-cutaneous flap after leech application for venous congestion is described and the current literature reviewed. Infection associated with leech therapy is a documented complication of leech application, with reported incidences ranging from 2.4 to 20 %. In some cases an infection of the wound developed with Aeromonas hydrophila, a Gram-negative rod that lives symbiotically in the intestines of the leech. Because of the risk of graft loss, early diagnosis and immediate initiation of an empirical intravenous antibiotic therapy with Piperacillin/Tacobactam or a third or fourth generation cephalosporins are essential even before results for sensitivity testing are received. An alternative is a short-term preemptive therapy with Cotrimoxazol or Ciprofloxacin during leech application.