The direct relationship between the knowledge of the manifestations of a certain disease and the frequency with which it is recognized is well exemplified by the morbid condition known as periarteritis nodosa. Between the time the disease was first described by Kussmaul and Maier1and the date of publication of Schreiber's2review of the literature in 1904, a period of thirty-eight years, only fifteen additional cases had been recorded in medical archives. The authentic cases gathered by Ophüls3in 1923 totaled seventy, which number represents an additional fifty-three reported within a space of nineteen years. The recent summary by Gruber4of the cases found in the literature includes a description of 117 reports, to which may be added one by Frommel,5four by Christeller,6one by Paul,7five by Arkin8and two by me, bringing the total up to 130, an increase of sixty within three years. The increased number