Purpose: Approximately 5-10 % of free flaps are subject to vascular compromise. Ninety percent of these flaps are compromised specifically by venous occlusion. Despite emergent salvage attempts, only 50 to 69 % of flaps survive. High salvage failures are presumably due to the fact that secondary ischemia associated with venous occlusion is more detrimental than that of the primary ischemia associated with an arterial occlusion. It has been suggested that muscles exposed to venous occlusion undergo hemorrhage of capillary beds due to increased intravascular hydrostatic pressures as well as increased interstitial edema from increased infiltration of neutrophils both resulting in cellular pressure necrosis. Moreover, we feel that external compression leads to a gradual and non-complete collapse of the venous system thus further compounding the increased hydrostatic pressures with persistent trickling of injurious neutrophils. We hypothesized that secondary ischemia arising from partial venous occlusion would be more injurious than that of complete venous occlusion. Methods: Rat gracilis muscle flaps were elevated on their major vascular pedicles and exposed to 3 hours of either partial venous occlusion (n=6) or complete venous occlusion (n=6). Muscle viability (per NBT staining) and muscle edema (per wet to dry muscle weight ratio) were measured following 3 hours of venous occlusion and 21 hours of muscle perfusion. Results. Baseline gracilis muscle venous pedicle flows (0.25 +/- 0.03 ml/min) were decreased by approximately 70 % in the partial venous occlusion group (0.08 +/- 0.01 ml/min) and by 100 % in the complete venous occlusion group (0 ml/min). Muscles exposed to partial venous occlusion demonstrated decreased muscle viability (6.7+/-5.4 %) when compared to muscles exposed to complete venous occlusion (24.4 +/- 8.9 %; P = 0.002). Also, muscles exposed to partial venous occlusion demonstrated increased muscle edema (3.7 +/- 0.2) when compared to muscles with complete venous occlusion (3.3 +/-0.2, P=0.047). Conclusion. We have observed that muscles exposed to partial venous occlusion (approximately 70 % venous flow occlusion) were more vulnerable to ischemic injury than muscles exposed to complete venous occlusion (100 % venous flow occlusion). These findings may help clarify the mechanisms of secondary venous ischemia associated with poor salvage rates.