The relationship between clear-cell syringoma and diabetes mellitus is well established. We present a case of clear-cell porocarcinoma in a diabetic patient. The lesion consisted of a 5-cm nodule on the lateral aspect of the left leg. Histopathologically, the neoplasm was composed of irregular aggregations of neoplastic cells with striking clear-cell appearance, showing features of ductal differentiation. The clear-cell appearance of neoplastic cells was due to glycogen accumulation within their cytoplasm. Immunohistochemistry and ultrastructural studies also supported the diagnosis of a neoplasm with sweat ductal differentiation. Enzyme histochemical reactions for phosphorylase immunoreactivity on fresh, unfixed sections of the neoplasm demonstrated that this immunoreactivity was remarkably decreased. Some adnexal neoplasms of the skin mostly composed of clear cells may be cutaneous markers of diabetes mellitus. Phosphorylase activity deficiency in diabetic patients may be responsible for glycogen accumulation in neoplastic cells resulting in clear-cell appearance of these neoplasms.