1. The issue of hypochondriasis has been confused through its identification, on one hand, with somatic delusions in psychoses and, on the other, with any and all somatic disorders of functional origin. This study supports the thesis that there exists a clinical nosological entity, called hypochondriasis, as here defined.2. Hypochondriacal patients, with very few exceptions, have numerous complaints. These involve organs, systems of organs, the whole or parts of the body not referable to any special organs. In contrast to their usual diffuseness, the complaints in a few of our patients are circumscribed warranting the term monosymptomatic hypochondriasis.3. The only common and very outstanding characteristic of hypochondriacal individuals is their unusual, peculiar hypochondriacal attitude towards health and sickness. The consistency of such an attitude and its persistence more or less overtly throughout life leads one to speak of constitutional hypochondriasis. However, aside from the hypochondriacal attitude, both male and female patients with hypochondriacal reactions do not, all of them, otherwise show distinctly characteristic personality traits. They rather present a rich collection of varied human specimens.4. It is granted that in the hypochondriacal reactions of our patients, their special personality make-up played the all-prevalent part. Yet, from our material it is also clear that environmental influences, including iatrogenic factors of long and short duration, equally contributed their share.