Hypochondriasis and OCD differ conceptually in the degree to which the patient's disease concerns are experienced as an intrusive mental event or a reasonable psychological response to a realistic health threat, in the degree to which the ideation is resisted, and in the presence of somatic sensations and medical help-seeking. There are, however, some similarities between the conditions, including the development of excessive, stereotyped, repetitive behaviors in an attempt to allay their anxiety. Empirical data on the degree of overlap between the conditions are too limited to permit definitive conclusions. The little that we do know, however, suggests that (1) the prevalence of OCD in hypochondriasis is probably elevated, but not extraordinarily so; (2) the prevalence of hypochondriasis in OCD is unknown; (3) fears about disease, illness, and injury are one of the more common forms of obsessions seen in OCD; and (4) there are several ill-defined and largely unexplored conditions, such as disease phobias, which appear to be very similar to both OCD and hypochondriasis. Clinical experience suggests that there may be a subgroup of hypochondriacal patients who are closer to the anxiety disorders in general and to OCD in particular. This subgroup might respond to the newer, antiobsessional, serotonin reuptake blocking agents.