Melanonychia, which is characterized by brown or black pigmentation within the nail plate, includes heterogeneous conditions such as pigmented nevus, subungual melanoma and lentigo. We treated two cases of subungual melanoma in situ. One case was a 58-year-old woman who suffered from a malignant melanoma in situ of the left third fingernail, who had also suffered from melanonychia of the fingers for more than 30 years. She had a past history of carcinoma of the uterine cervix. The other patient was a 42-year-old man, who suffered from a malignant melanoma in situ of the right fifth fingernail. He had a past history of carcinoma of the stomach for which he had undergone surgery 2 years earlier. Both cases were accompanied by Hutchinson's sign on the fingertip skin, and the presence of this sign led to the correct diagnosis of subungual melanoma in situ. Judging from previously reported cases, it is unlikely that patients with malignant melanoma have an increased risk of carcinoma of the uterine cervix or of the stomach.