In this series, a psychotic reaction in one twin disturbed the twin relationship. When attempts of the other twin to compensate for this disturbance were not successful, an alternating psychotic reaction developed. Most often the psychoses were of paranoid or depressive type, thus emphasizing the frequent attributing of cause for the illness to the other twin and the acceptance of responsibility by that twin with subsequent guilt and depression.There was evidence of strong rivalry between the twins during illness as well as during relatively well periods; this rivalry led to a sharing of medical and family care. The second twin was able unconsciously to exact as much help from his environment as the first. The unusually strong identification observed in normal twins appeared to carry over into illness. Ego integrity of one twin may depend on the integrity of the ego of the other.