Subungual melanoma is a rare subtype of melanoma that usually originates and spreads from the nail matrix. Because of its poor prognosis and short matrix-to-bone distance, amputation has been traditionally performed. Recently, conservative surgery has been attempted for early subungual melanoma, but the evidence supporting this practice is sparse. As little is known about the progression pattern of subungual melanoma, further advances on the subject may provide better guidance on the optimal surgical approach. Histopathology slides, clinical records, and photographs of 23 cases of subungual melanoma were reviewed. For all cases, each area of the nail unit-proximal nail fold, nail matrix, nail bed, and/or hyponychium-in longitudinal sections was available for histological examination. Growth pattern, dermal invasion, and thickness were assessed in each area of the nail unit. There were five cases of melanoma in situ. Eighteen cases showed dermal invasion in at least one area of the nail unit. There were no cases showing dermal invasion in the nail matrix area only. In four cases, dermal invasion involved areas of the nail unit other than the nail matrix. In 14 cases, dermal invasion involved the nail matrix area as well as other areas of the nail unit. Except for one case, the nail matrix area showed thinner dermal invasion compared with dermal invasion in other areas of the nail unit. In conclusion, dermal invasion of subungual melanoma in the nail matrix area tends to occur later than other areas of the nail unit. Longitudinal incisional biopsy is necessary to accurately evaluate melanoma invasion. The findings of this study suggest that conservative surgical treatment for early subungual melanoma may be justified as the nail matrix area, an area of thin dermis and close proximity to the underlying bone, appears to be more resistant to invasion.