Two groups of obsessive-compulsives who failed to respond to treatment, although they complied with its demands, are examined. Patients of the first group held a strong conviction that their fears were realisitic; the second group was composed of patients who manifested severe depression. In the first group, patients habituated within sessions but not between sessions. Neither form of habituation was shown in the second group. By contrast typical obsessive-compulsive patients who benefited from behavioral treatment showed both forms of habituation. It is proposed that different processes underlie the two forms of habituation. one autonomic and the other cognitive, involving different areas of the brain. The occurrence of within-session habituation is seen as a necessary, but not a sufficient condition for between-session habituation. Implications for treatment are briefly discussed. © 1979.