Ungual melanoma is known to be frequently misdiagnosed, and the primary misdiagnosis of ungual melanoma includes onychomycosis. We report a very rare case of onychomycosis concealing ungual melanoma in situ. A 52- year-old male patient presented with a yellow to brown thickened left great toenail that had existed for 10 years. In the front view, Hutchinson's sign which refers periungual extension of brown-black pigmentation from melanonychia was also observed on his hyponychium. First, fungal infection of his toenail was confirmed with positive KOH result. Next, a nail biopsy after nail avulsion was done to assess Breslow depth and to determine surgical margin. And ungual melanoma was diagnosed with immunohistochemical stains. Non-amputative wide local excision with 5 mm surgical margin followed by skin grafting was done Unfortunately in three years ungual melanoma in situ has recurred 2 times and then progressed to invasive malignant melanoma, so he was referred to the Department of plastic surgery for further invasive surgical treatment and regularly monitored to check the recurrence. According to a prior retrospective study, 52% of ungual melanomas were clinically misdiagnosed. Thus, proper diagnosis of ungual melanoma is very important for the patients' better prognosis. It would be important for physicians to examine the nail plate as well as the underneath structures for patients with nail disease including onychomycosis.
