Longitudinal erythronychia is a frequent nail presentation with a limited differential diagnosis. This clinical entity may be divided into cases that involve one (localized) or multiple (polydactylous) nails. The different presentations have distinct differential diagnoses and workups yet often share a common pathogenesis. Localized longitudinal erythronychia most commonly represents onychopapilloma, yet malignancies may present identically. Therefore biopsy may be required. Polydactylous longitudinal erythronychia usually coincides with a regional or systemic cause. Occasionally, it may herald an important underlying disease. A thorough understanding of the pathogenesis, clinical presentations, and possible diagnoses is necessary for successful evaluation and management.