Purpose: The purpose of this study was to consider the relationship between the ratio of deep tissue including muscle to thigh at donor sites and the possibility of performing primary closure of donor site. Methods: The subjects were 74 patients who had harvesting of anterolateral thigh (ALT) free flap from June 2005 to June 2011. Primary closure was possible for 65 but not possible for 9. All received CT angiography of lower extremity before their operations. We measured circumference and cross-sectional area of thigh and deep tissue including muscle at the reference point. Using the measured data, we examined the ratio of circumference as well as cross-sectional area of deep tissue including muscles to thighs. Results: For whom primary closure was possible, the ratio of deep tissue including muscle's circumference to thigh's at the reference point was 0.83 ± 0.07 on average, and the ratio of cross-sectional area was 0.68 ± 0.11. For whom primary closure was not possible, the ratio of circumference was 0.89 ± 0.06 on average, and the cross-section areas was 0.8 ± 0.07. The average width of flap for those with primary closure was 64.9 mm and without primary closure was 84.4 mm. There was statistical significance in ratios of circumference and cross-sectional area between primary closure and without primary closure. Conclusion: Primary closure of donor site when performing ALT free flap gets increasingly difficult as the ratio of deep tissue including muscle in the thighs increased. Such information prior to the procedure will be helpful in determining flap design and finalizing the operation plan. © 2013 Wiley Periodicals, Inc. Microsurgery, 2013.