We report an apocrine adenocarcinoma of the left axilla associated with hamartomatous apocrine gland hyperplasia of both axillae. The patient, a 69-year-old man, presented with no symptoms or complaints other than an oval mass felt in the left axilla. The mass was resected and histopathological examination revealed a papillary apocrine adenocarcinoma located within hyperplastic apocrine glands. Because gallium scintigraphy performed after the operation still showed bilateral abnormal uptakes, skin and subcutaneous tissues of the bilateral axillary areas were resected. Histological examination demonstrated marked multilobular hyperplasia of the apocrine glands. These hyperplastic glands did not show distinct atypia, and there was no evidence of tumor remnants in the left axilla. The patient has shown no signs of local recurrence or metastasis at 20 months' follow-up. To our knowledge, this is the first case of malignant transformation of hamartomatous apocrine gland hyperplasia (apocrine gland organic hamartoma or apocrine nevus).