In a previous paper by one of us (H. O. C.1) the separate character of benign and malignant acanthosis nigricans was stressed. Differences between the two types exist with regard to the age of the patient at the onset of the dermatosis, the course, and the activator of the disorder. It was also pointed out that there may be in the genetic background of the two types significant differences which could furnish additional evidence for the assumption that benign and malignant acanthosis nigricans are separate diseases. The result of our genetic studies has confirmed our early impression. Since some controversy over the necessity of recognizing different types of acanthosis nigricans has arisen, we shall briefly define benign and malignant acanthosis nigricans before reporting our genetic data on these forms of the dermatosis. Pseudo acanthosis nigricans, a third variety, will also be described, and the reasons