ImportanceThe overdiagnosis of melanoma in situ (MIS) is well documented. There is limited evidence on the rate of local recurrence of the non-lentigo maligna (non-LM)/non-acral lentiginous melanoma (non-ALM) subtypes.ObjectiveTo investigate local recurrence and prognosis in non-LM/non-ALM MIS, the histopathological clearance of the excisional biopsy margins, and the association with the size of wide excision margins.Design, Setting, and ParticipantsThis retrospective cohort study included patients with non-LM/non-ALM MIS diagnosed from 1991 to 2023 who were followed up for at least 1 year at the Skin Cancer and Melanoma Unit of Andreas Sygros University Hospital in Athens, Greece. Patients with a history of invasive melanoma or a histopathological diagnosis of LM or ALM in situ were excluded. Median (IQR) follow-up was 5.2 (2.9-7.9) years. Deidentified data on patient demographics and clinical characteristics, including the histopathological clearance of margins of the initial excisional biopsy and the size of margins of the wide excision, were obtained from medical records.Main Outcomes and MeasuresThe primary outcomes were local recurrence, metastasis, and melanoma-specific survival.ResultsA total of 401 patients (214 [53.4%] women) with 403 non-LM/non-ALM MIS and a median (IQR) age of 52 (40-62) years were included in the analysis. MIS was frequently located on the trunk (201 lesions [49.9%]), followed by the lower extremity (99 [24.6%]), the upper extremity (71 [17.6%]), and the head and neck (32 [7.9%]). All lesions were initially treated with excisional biopsy, followed by wide excision for 372 (92.3%). There was only 1 local recurrence in a patient with involved margins at the excisional biopsy who did not undergo wide excision and developed an invasive melanoma 14 months later. Thirty lesions in 30 patients had clear excisional biopsy margins with no wide excision and had no recurrence at a median (IQR) follow-up of 8.1 (4.1-12.9) years. In 23 patients with 23 lesions that had wide excision with narrower than the standard 0.5-cm margins (mean [SD] margin size, 0.36 [0.07] cm), no recurrences were found at a median (IQR) follow-up of 4.3 (2.7-6.2) years. During follow-up, 6 patients (1.5%) developed a lesion suspicious for recurrence near the excision scar, which was excised and showed nevus or solar lentigo on histopathology. No patients had metastasis or melanoma-specific death.Conclusions and RelevanceThis cohort study found that diagnostic excisional biopsies with clear margins may be sufficient for treating MIS; however, larger studies are necessary.