This article reviews the literature on the clinical implications of a partial response to use of antidepressants in depression treatment. Patients who fail to reach premorbid levels of symptom remission are more vulnerable to relapse, work impairment, & suicide. Partial response may be one form of treatment resistance. Up to 20% of patients with major depression may be resistant to treatment with antidepressant medications. Future directions for research are noted. (27 Refs.)