A follow-up study of 208 manic-depressive conditions is reported. Analysis of such conditions permits the following summary.Positive heredity is common—chiefly depressions, and more commonly in the direct line of heredity. This shows more consistently in the manic cases. Characteristic personality type is outgoing. Unexplainable psychosis is rare (20 per cent). Most have disturbing circumstances at the time of onset. Oldest and youngest siblings may more frequently develop psychoses. Somatic handicaps frequently contribute to or help to precipitate the illness. Abrupt onset is rare except in manics. The condition usually requires one to six months to develop. Suicidal risk or attempt occurred in 30 per cent of all cases. The affective picture is often mixed, not purely as sadness or elation. Psychoneurotic (not often as anxiety), schizophrenic, and arteriosclerotic components are frequent (79 cases). (33.1 per cent). Thinking disturbances are present in one-fourth of the cases (manic and depressive phases). Capacity for rapport is fair or good in 41 per cent of the manics and in 70 per cent of the depressions. Some insight is present in 47 per cent of the manics and in 54 per cent of the depressions. Remissions of 10-20 years are common (67 cases, 32.2 per cent). Depression may begin early, but rarely (3.8 per cent, 8 cases), or late (65+, 11 cases, 5.2 per cent). The largest incidence occurs in the decade 45-55. Course is a matter of months; on the average 2½ months in the hospital. Three-fourths of the cases are well or improved at the time of discharge; 93 per cent recover from the first attack. Chronic cases are rare (16, 7.7 per cent). Cyclothymic cases tend oftener to chronicity. Single attacks are rare (21 per cent). Recurrence is likely. Long-term follow-up convinces us that caution is necessary in pronouncing recovery for the future. Seventy-nine per cent had recurrences. This is particularly true for cyclothymic cases. Many variations in course and phases occur. Depression following elation is more common than vice versa. Depression almost always lurks behind elation. Manic patients may suicide. Depressive or paranoid content occurred in 14 of the manic cases (77.7 per cent). Depressive cases may show definite manic features (spurts of elation in the morning, rhyming and flight with sadness). Single attacks of manic psychosis are very rare. Single attacks of depression occurred in 40 cases (19.2 per cent). Fourteen of these became chronic (35 per cent of 40 and 6.8 per cent of 208). The unfavorable depressions may develop a schizophrenic picture. Deterioration may occur in depression. Stupor in depression occurs—catatonic in appearance. First attacks on the average last 6½ months. Recovery from first attack is almost invariable (93 per cent). Twenty-one per cent had single attacks; 79 per cent had recurrent illnesses; 63.5 per cent had third attacks, and 45.1 per cent had fourth attacks. Attacks tend to become prolonged as they recur, especially after 45. Best prospect for recovery for patients having first attack is between 21 and 30, poorer after 40, worst after 50. Recovered patients show small number of attacks (2.7) and shortening of the course in subsequent attacks. The more frequent the attacks, the less chance for ultimate recovery. Depth of psychosis is not significantly important. Negative heredity is not an outstanding factor in recovery. Out-going make-up is a factor in recovered cases.