Fourteen subjects with illness phobia, a subtype of hypochondriasis, were compared with an equal number of subjects with panic disorder who had been matched for age and sex. The illness phobic subjects differed from panic subjects in not having spontaneous panic attacks or agoraphobic symptoms, the characteristic features of panic disorder. The onset of illness phobia was related to experience with illness in half the subjects. Half of the illness phobic subjects also had family histories of anxiety disorders. The results suggest that illness phobia is distinct from panic disorder and that it is a disorder in which environmental and genetic factors are etiologically important.