Acanthosis nigricans is characterized by the formation of hyperpigmented, velvety plaques that occur mainly on the lateral aspects and nape of the neck and in the axillas and groins. Microscopically, these plaques show papillary hypertrophy, waviness of the surface, thickening of the epidermis, hyperkeratosis, and hyperproduction of melanin pigment. Around the plaques of acanthosis nigricans single, nonpigmented papilloepitheliomas, pendulous fibromas, and verruciform excrescences are often seen. Two distinct types of acanthosis nigricans are recognized. In the first (malignant or adult) type, malignant tumors of internal organs can invariably be found. In the second (juvenile or benign) type the plaques appear at an early age, and no malignant lesion is demonstrable. Interestingly, however, in two reported cases one parent of each patient died early from bronchogenic carcinoma. Patients with acanthosis nigricans of the benign or juvenile type are usually excessively obese. Some of the patients have glycosuria at irregular intervals, some