To study factors associated with different mood disorders in old age, researchers need clear and meaningful definitions. "Major depression" is too broad and heterogeneous a category, and the boundaries of "dysthymia" are ill-defined, yet epidemiologic studies have focused on these disorders. Depressions in old age are commonly associated with medical conditions; prevalence rates of depression in cases of stroke, Parkinson's disease, dementia, and disabilities (all much commoner in old age) range upward from about 20%. Depressions are attributed to both psychological and biological reactions. Mania, too, can be precipitated by cerebral and other medical factors. The proportion of psychiatric inpatients who have depressions with melancholic and/or psychotic features is higher among elderly subjects, and this has been linked with white matter lesions and other brain changes that become commoner in old age. The prognostic relevance of these brain changes, and implications for treatment of mood disorders, require clarification.