• In a family study of 133 probands with major depression and 82 normal control subjects, and 1,518 of their first-degree relatives, we found a substantial inverse relationship between the age of onset of major depression in the probands and the risk of major depression in their relatives. The relatives of probands whose onset of major depression occurred when they were younger than 20 years of age had the highest risk of major depression, compared with the relatives of probands who had later ages of onset or with the relatives of normal subjects. Probands with an age of onset of 40 years or more had familial loading that was only slightly higher than the families of normal control subjects. Our statistical methods enabled us to examine the relationship of the ages of onset in the probands and their relatives while accounting for possible confounding factors. More studies will be needed to sort out secular changes in the rates of the occurrence of major depression among young persons (cohort effect) from the high familial loading of major depression that has its onset in childhood and adolescence, and to determine whether the specificity of transmission of early-onset depression is the result of a single homogeneous disorder.