The follow-up is the great exposer of truth, the rock on which many fine theories are wrecked and upon which better ones can be built; it is to the psychiatrist what the postmortem is to the physician. —P. D. Scott THE CAUSE of most medical and psychiatric illnesses is unknown. Even without knowledge of etiology, however, the physician can diagnose, predict and treat. To accomplish this he may rely to some extent on personal experience, but this alone is rarely sufficient. A knowledge of "the literature" is usually essential. Especially helpful are follow-up studies: published reports of groups of patients having similar symptoms followed over time to determine both "natural" outcome and outcome as influenced by treatment. To illustrate their usefulness we have reviewed follow-up studies pertaining to one illness, obsessive compulsive neurosis, and summarized the findings. Of the 13 studies reviewed,