This study entailed direct stereomicroscopic observations of rat abdominal island flaps and the sequence of vascular events that occurred during and after occlusion by clamping of the individual femoral vessels for varying time intervals of up to 16 hours. The vascular changes and the status of the circulation in the flap vasculature, including the timing of complete hemal stasis and resumption of flow, following the application and subsequent release of the vessel clamp were observed and correlated with flap viability and behavior. This study also included a grading system of the vascular changes observed under a stereomicroscope. From these findings, it is suggested that in vivo stereomicroscopy may serve as a new and simple clinical tool for the early diagnosis of thrombosis at the site of microvascular anastomosis of a free flap transfer and may define time limits during which the anastomosis can be successfully corrected and the flap salvaged.