We reviewed 43 free flaps in 40 consecutive patients (1999-2002) in which the implantable venous Doppler was used to listen to recipient vein outflow after vascular anastomosis. The Doppler allowed us to detect cessation of blood flow in the recipient vein before vascular thrombosis occurred in 9 of the free flaps. All of the cases were re-explored and examined directly for the cause of the cessation of flow that would have ultimately led to thrombosis. In 5 of the cases, the cause was a kink in the vein. Repositioning the vein to get rid of the kink salvaged all 5 flaps. In 2 cases, the cause was low flow in the flap at the time the vessel clamps were let go. In spite of patent anastomoses, these flaps were lost because there was not enough flow to sustain them. One case was found to have compression of the vein after insetting which was successfully corrected, and the final lost signal was attributable to arterial vasospasm. Of these 9 cases, none had a loss of flow because of technical anastomotic errors. Through these pre-thrombotic conditions that we detected with the implantable venous Doppler, we have begun to understand why vascular thrombosis may ultimately occur.