A 71-year-old man presented to the authors' clinic for evaluation of a red line under his right thumb. He noticed a red streak develop during the past year. It slowly grew in width and become more prominent in color (Figure 1). It did not cause pain. He delayed presentation because he perceived it to be only a cosmetic issue. Medical history included a metastatic atypical carcinoid tumor to the liver, lung, and the bone diagnosed 9 years ago. He had undergone multiple debulking surgeries and was currently taking octreotide and zoledronic acid. He had not started any new medications in the past 2 years. Review of systems was unremarkable. On physical examination, the right thumb nail was noted to have a red streak that began at the distal matrix. The line ended at the distal nail plate with distal disintegration and subungual hyperkeratosis. A biopsy was performed through the nail plate. The site removed by biopsy included the area in which the erythronychia visibly started, as well as the preceding normal nail matrix. The ventral nail plate was noted to have a groove of thinning, with slight purple discoloration. The nail bed/matrix was red in a linear pattern and no clinically apparent hyperkeratosis was noted. The matrix/bed sample was sent for pathologic evaluation. Notable findings included an acanthotic epidermis with some enlarged nuclei (Figure 2). Mild capillary dilatation was present in papillary dermis. Focal solar elastosis in the distal portion of the nail bed was identified. In situ hybridization for low- and high-risk human papillomavirus was negative. An immunohistochemical study using a panmelanocytic cocktail (HMB45, anti-MART1, anti-tyrosinase) failed to reveal any melanocytic lesion. Perl's iron stain was negative. Metastatic carcinoid or primary squamous cell carcinoma were not identified.