Seroma formation is a common complication following anterolateral thigh (ALT) flap harvesting. This study aimed to evaluate whether age or the fascia harvest area is more strongly associated with postoperative seroma formation.

We retrospectively reviewed 51 ALT flaps in 50 patients at Aichi Medical University between April 2021 and July 2025. One patient underwent a bilateral harvest. All donor sites were closed primarily, and cases of drain self-removal or skin graft closure were excluded. Seroma was defined as donor-site fluid collection confirmed by ultrasound, CT, or aspiration within 2 months postoperatively. Univariate analyses and Firth’s penalized logistic regression analyses were performed.

Five flaps (9.8%) developed seroma in patients aged ≥ 70 years (5/20, 25%) compared with none in patients aged < 70 years (0/31, 0%; p = 0.0066). The median fascia harvest area (172.5 vs. 112.5 cm²; p = 0.526), BMI (22.9 vs. 22.6; p = 0.913), and drain removal timing (POD 7 vs. POD 7; p = 0.901) did not differ significantly between groups. The drain output within 24 h before removal was higher in the seroma group (21 vs. 11 mL; p = 0.0058). Firth’s penalized logistic regression confirmed age as an independent predictor (OR 1.13 per year; 95% confidence interval [CI], 1.01–1.41; p = 0.030). Bootstrap validation demonstrated that the age effect remained stable across resampled datasets despite the small number of seroma events.

An older age was significantly associated with donor-site seroma after ALT flap harvest, whereas the fascia harvest area and BMI were not. Although drain output was higher in the seroma group, this may reflect early seroma formation rather than serve as a causal predictor. Elderly patients may require closer postoperative monitoring to reduce donor site complications.