UNLABELLED: Anterolateral thigh and fibula free tissue transfers are widely used methods of reconstruction in head and neck malignancy, yet the risk factors for donor site morbidity are not well studied. The objective of this study was to characterize anterolateral thigh and fibula free flap donor site morbidity and identify associations with social determinants of health. In 116 patients who underwent anterolateral thigh or fibula free tissue transfer for the reconstruction of head and neck oncologic defects, the presence of a social determinant of health did not lead to an increased rate of donor site morbidity. However, having two or more social determinants was associated with an increased risk of flap failure, and the presence of social determinants may reduce likelihood of patient completion of recommended postoperative physical therapy. IMPORTANCE AND PURPOSE: Anterolateral thigh and fibula free tissue transfers are widely used methods of reconstruction in head and neck malignancy, yet the risk factors for donor site morbidity are not well studied. The objective of this study was to characterize anterolateral thigh and fibula free flap donor site morbidity and identify associations with social determinants of health. DESIGN AND METHODS: This retrospective cohort study examined 116 patients from December 2015 through June 2022 who underwent head and neck oncologic reconstruction with anterolateral thigh or fibula free flaps at a single institution tertiary care center. The one year postoperative period was inspected, and data on outcomes of donor site morbidity, flap failure, and physical therapy completion were collected. RESULTS: 116 patients (91 male and 25 female) met inclusion criteria. Thirteen (17.6 %) of the 74 anterolateral thigh patients and 22 (52.4 %) of the 42 fibula patients developed any donor site complication within the one year postoperative period. 53.4 % of the population had at least one documented social determinant of health. The presence of a social determinant was not associated with an increase in adverse donor site outcomes compared to patients without documented social determinants (OR = 0.54, 95 % CI 0.24-1.20, p = 0.135). However, 14.3 % patients with two or more social determinants of health experienced flap failure compared to 1.1 % the those with one or zero social determinants (p = 0.012). Within the subpopulation that had further post-discharge physical therapy recommendations, 72.7 % of patients with social determinants of health failed to complete this therapy compared to 0 % of patients with no social determinants (p = 0.026). CONCLUSIONS AND RELEVANCE: The presence of a social determinant of health did not lead to an increased rate of donor site morbidity. However, having two or more social determinants was associated with an increased risk of flap failure, and the presence of social determinants may reduce likelihood of patient completion of recommended postoperative physical therapy.