Two cases of cudden non-traumatic death associated with physical exertion in identical twins. The one observation concerned a famous rugby player who died immediately after a competitive game. Autopsy revealed cardiac hypertrophy and advanced myocardial fibrosis, excess lymphoid tissue, a larghe thymus gland with active Hassals bodies and hydronephrosis of the left kindney. A twin brother of the deceased died at about the same time during swimming. The second instance was studied by O'Brien. It refers to a man age 26 who died during physical work on the farm (pulling sugar beets). Post mortem examination showed a hemmorrahagic cyst with a pigmented wall and a thrombosed aneurysm of the left middle cerebral artery which had leaked. An identical twin brother of this man had died a short time previously while playing tennis.