Corticosteroids such as prednisone are commonly prescribed for a variety of illnesses mediated by the immune system. This paper reviews the available literature on mood symptoms during corticosteroid treatment. Few studies have used well-recognized measures of symptoms or clearly defined diagnostic criteria to characterize such mood changes. The limited data available suggest that symptoms of hypomania, mania, depression, and psychosis are common during therapy. Symptoms appear to be dose dependent and generally begin during the first few weeks of treatment. Risk factors for the development of mood instability or psychosis are not known. The similarities of the psychiatric symptoms resulting from corticosteroid treatment to the symptoms of bipolar disorder are discussed.