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Importance: Melanoma guidelines recommend surgical excision with 10-mm margins for T1 melanoma. However, this procedure may be problematic at sites close to critical structures such as the scalp, face, external genitalia, acral, periumbilical, and perineal areas., Objective: To compare outcomes of wide (10-mm margins) vs narrow (5-mm margins) excision in patients with T1a melanoma near critical structures., Design, Setting, and Participants: This cohort study was a retrospective comparison of 1341 consecutive patients aged 18 years or older from the National Cancer Institute of Milan, Italy, diagnosed between 2001 and 2020 with T1a cutaneous melanoma close to critical structures who accepted wide excision vs narrow excision., Exposures: Local recurrence and melanoma-specific mortality (MSM) rates with 5-mm vs 10-mm excision margins., Main Outcomes and Measures: The primary aim of the study was to ascertain whether a narrower (5-mm) vs wider (10-mm) excision margin was associated with local recurrence and MSM. The secondary aim was to compare the need for reconstructive surgery in the groups defined by excision margin width. Between April 28 and August 7, 2022, associations were assessed by weighted Cox and Fine-Gray univariable and multivariable models., Results: A total of 1179 patients met the inclusion criteria (median [IQR] age, 50.0 [39.5-63.0] years; female, 610 [51.7%]; male, 569 [49.3%]). Six hundred twenty-six patients (53.1%) received a wide excision (434 [69.3%] with linear repair and 192 [30.7%] with flap or graft reconstruction) and 553 (46.9%) received a narrow excision (491 [88.8%] with linear repair and 62 [11.2%] with flap or graft reconstruction). The weighted 10-year MSM was 1.8% (95% CI, 0.8%-4.2%) in the wide group and 4.2% (95% CI, 2.2%-7.9%) in the narrow group; the weighted 10-year local recurrence rate was 5.7% (95% CI, 3.9%-8.3%) in the wide group and 6.7% (95% CI, 4.7%-9.5%) in the narrow group. Breslow thickness greater than 0.4 mm (subdistribution hazard ratio [sHR] for 0.6 vs 0.4 mm, 2.42; 95% CI, 1.59-3.68; P < .001) and mitotic rate greater than 1/mm2 (sHR for a single increment, 3.35; 95% CI, 2.59-4.32; P < .001) were associated with worse MSM. Multivariable analysis showed that acral lentiginous melanoma, lentigo maligna melanoma, and increasing Breslow thickness were associated with a higher incidence of local recurrence., Conclusions and Relevance: The study's findings suggest that local excision with 5-mm margins for T1a melanoma may not be associated with an increased risk of local recurrence. Breslow thickness greater than 0.4 mm, mitotic rate greater than 1/mm2, and acral lentiginous melanoma and lentigo maligna melanoma subtypes were associated with a higher risk of recurrence. These findings may be useful for future melanoma treatment guidelines.