A systematic register of twin births, covering the southernmost province of Sweden from 1870 onwards, was erected at the Psychiatric Department of the University of Lund in the late forties; it is thus the oldest of the Scandinavian twin registers. The register contains 20 000-25 000 twin pairs, unselected for survival. By confrontation with some 15 000-20 000 consecutive Cases of acute concussion of the brain, from surgical wards, and a similar number of alcoholics, and of patients with peptic ulcer, a number of MZ pairs discordant for each of these diagnoses were extracted and thoroughly examined in a psychiatric and psychological study of nervous sequels, organic deterioration, and the influence of stress.