Background: Longitudinal melanonychia (LM) may occur as a result of nail apparatus melanoma. Knowledge of etiology plays an important role in the management of LM. Objectives: The study is aimed to compare the diagnosis of LM in different age groups. Methods: We collected 63 cases (45 adults and 18 children) with LM who underwent nail matrix biopsy or excision in a 21-year cohort and assessed their clinicopathological features. Results: Melanomas in adults and children were 40% and none, while nevi accounted for 15.6% in adults and 94.4% in children. There was a statistically significant difference between the average age at diagnosis for melanoma (54.5 ± 13.3 years) and nevus (15.2 ± 18.5 years). Logistic regression related the occurrence of melanoma to older ages with a relative risk of 1.2 compared to nevus, but no cutoffs between age groups could be defined between LM-associated nevus and melanoma. Conclusion: The adult group has a significantly higher risk of melanoma, while children with LM show mostly nonmelanoma etiologies. Tissue proof is more warranted in adult cases, and it is needed in selected cases of children with LM.

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