There are several advantages for groin flap, but its small and unpredictable vessels of pedicle have made it to lose its initial popularity. Although it would be ideal flap when it is focused on its useful advantages such as relative larger size, low donor site morbidity and possible bone graft, there have been few studies for prognostic factors for successful groin flap. Authors intended to determine prognostic factors which are relative with success of free groin flap. From January 1985 to December 2007, 107 patients who underwent groin flap for reconstruction of extremities were selected consecutively. Univariate and multivariate analysis were performed to determine prognostic factors which were related with success of groin flap. Eighty of 107 (74.8%) flaps survived. There was significant difference in success rate according to the recipient site. Nineteen of 20 cases (95%) survived in upper extremities, but 61 of 87 cases (70.1%) survived in lower extremities, which was statistically significant (p=0.022). Univariate analysis showed that mean diameter of donor veins was significantly larger in success group (p=0.021). Groin flap is recommended for reconstruction of upper extremities than lower extremities. It is thought to be critical that surgeons try to match vessel diameters between donor and recipient site.