This article reviews the challenge of diagnosing depression in patients with cancer. Major depression and depressive symptoms, although commonly encountered in medical populations, are frequently underdiagnosed and undertreated. This is especially true for patients with cancer in whom the diagnosis of major depression is clouded by neurovegetative symptoms that may be secondary to either cancer or depression. Well-established biological markers for major depression are proposed as diagnostic adjuncts in patients with cancer. Studies using biological markers in depressed patients with and without cancer are reviewed, and the implications of diminished immune function in depressed patients with cancer are discussed. The limited database on treatment of depression in patients with cancer also is reviewed. Treatment of depression in these patients improves their dysphoria and other signs and symptoms of depression, improves quality of life, and may improve immune function and survival time. Guidelines for future research are proposed.