An acral lentiginous melanoma (ALM) is the most common type of cutaneous melanoma in Asians. However, it is unusual to encounter a patient showing only the histologic features of an ALM in situ. Here we describe clinical and histologic features of nine cases of ALM in situ including immunohistochemical staining with anti-S 100 protein and HMB-45. All the patients had a long clinical history. Clinically, the lesions were characterized by a longitudinal pigmented streak in the nail plates, black pigmentation on the proximal or lateral nail fold, and an irregular border and variegated pigmentation on the sole or thumb. Total resections of the lesions were performed in all patients. All lesions, in both biopsy and excisional specimens demonstrated more melanocytes generally located in the basal layer of the epidermis. There was no dermal invasion. No recurrence of the disease had occurred in any patient after a follow-up period of between 6 months to 12 years after surgery. These results suggest that ALM can evolve slowly over many years. However, it is unusual to encounter a patient showing only the histologic features of an ALM in situ. Here we describe clinical and histologic features of nine cases of ALM in situ including immunohistochemical staining with anti-S 100 protein and HMB-45. All the patients had a long clinical history. Clinically, the lesions were possession by profiling in the nail plates, black pigmentation on the proximal or lateral nail fold , and an irregular border and variegated pigmentation on the sole or thumb. Total lesions of lesions were performed in all patients. All lesions, in both biopsy and excisional specimens demonstrated more melanocytes generally located in the basal layer of the epidermis. No recurrence of the disease occurred in any patient after a follow-up period of between 6 months to 12 years after surgery. These results s uggest that ALM can evolve slowly over many years.