It has been suggested by clinical observations that acute venous thrombosis following free-flap surgery results in more severe tissue injury than postoperative arterial thrombosis. This current study, in the pig model, was designed (1) to determine the rate of survival and the percentage area of flap survival in both cutaneous and myocutaneous flaps following a secondary venous ischemic insult, (2) to compare this with findings following a secondary arterial ischemic insult, and (3) to determine the reliability of laser Doppler flowmetry and dermofluorometry index in the prediction of necrosis outcome following both venous and arterial ischemic insults. A total of 116 flaps were elevated in 29 pigs. Data analysis documented a decrease in the rate of flaps surviving venous ischemia as compared with arterial ischemia or control and of arterial ischemia as compared with control. In contrast to our previous work, a lower rate of buttock flaps survived a secondary ischemic insult (of either arterial or venous etiology) than did the cutaneous component of the myocutaneous flaps. As expected, as the length of ischemia increased, the rate of flaps surviving decreased, as did the percentage area of survival. Although laser Doppler performed slightly better than dermofluorometry index, as measured on immediate reperfusion following secondary venous ischemia, both were relatively poor predictors of eventual survival or necrosis. Under conditions of immediate reperfusion following secondary arterial ischemia, dermofluorometry index proved to be a superior predictor of ultimate necrosis as compared with laser Doppler flowmetry.