BACKGROUND: Leeches are commonly used in reconstructive surgery for the treatment of venous congestion in flaps. Documented mechanisms of action are (1) injection of the anticoagulant hirudin; (2) active suction of blood; and (3) passive oozing of the bite wound. Even though the benefits of leeches in venous congestion are widely accepted, little is known about their effects in mixed arterio-venous insufficiency. METHODS: Thirty Wistar rats were randomized into three groups (n = 10 each), and 9 x 3 cm ischemic random skin flaps were elevated on the rat dorsum. Group 1 served as controls. In group 2, one leech was applied to the distal part of the flap and in group 3, three leeches were applied, one at a time, at 8 h intervals. Postoperatively, flap survival and perfusion were quantitated by daily planimetry and laser-Doppler imaging. RESULTS: The application of a single leech on postoperative day 7 did not lead to any statistically significant changes in total flap survival or tissue perfusion. Triple leech application, however, caused a significant decrease in flap survival of 6.6% at day 7 (3x leech versus control: 45.8% +/- 8.5% versus 52.4% +/- 8.5%, respectively) (P < 0.01) and a decline in flap perfusion of 13% (3x leech versus control: 71.3% +/- 16.6% versus 84.3% +/- 9.3%, respectively) (P < 0.01). CONCLUSION: The data demonstrate that the application of leeches in the setting of mixed arterio-venous insufficiency can be hazardous to flap viability. Pure venous congestion with an adequate arterial supply remains the only indication for controlled leech application in the clinical setting.