A major factor in evaluating and treating depression is the presence of comorbid medical problems. In this paper, the authors will first evaluate studies showing that medical illness is a risk factor for depression. The authors will review a series of randomized, controlled studies of antidepressant treatment in subjects with major depressive disorder (MDD) and comorbid medical illnesses (myocardial infarction, stroke, diabetes, cancer, and rheumatoid arthritis). Most of these studies report an advantage for an active antidepressant over placebo in improvement of depressive symptoms. The authors also will review a series of studies in which the outcome of antidepressant treatment is compared between subjects with MDD with and without comorbid medical illness. In these studies, subjects with medical illness tend to have lower improvement of depressive symptoms and higher rates of depressive relapse with antidepressant treatment compared with MDD subjects with no medical comorbidity. In addition, the authors will review hypotheses on the mechanism of the interaction between medical illness and clinical response in MDD. The paper will conclude that medical comorbidity is a predictor of treatment resistance in MDD.