The author describes a method of diagnosing periarteritis nodosa during life with the aid of the sigmoidoscope. The pathologic basis for this method is the high frequency of involvement of the intramural intestinal vascular branches of the mesenteric vessels. The characteristic appearance is represented by horizontal, linear thromboses of the submueosal, arterial branches, not obliterated by direct pressure. By means of a telescopic device and green filter, the lesion is clearly located within the vessel since the normal portion of the vessel can be seen proximal to the linear thrombosis and as a thin, almost bloodless hair line distal to it. Besides the linear lesion, nodular and "skip" or segmental types are also noted. The observations and diagnoses made during life are corroborated by careful necropsy studies supplemented by similar findings in 2 other cases of periarteritis nodosa.