1. Mathewson, F.A.L. and Taylor, W.J.R.: Prolonged P-R interval in apparently healthy people. Ass'n of Life Ins Med Dir of America 36; 44, 1952. Of 3,983 members of the Manitoba Follow-up Study, 57 exhibited a P-R interval exceeding 0.20 secs. over an average of 9 years follow-up. Considerable variation in the P-R interval was observed on one or more occasions in 89% of the cases. There was no death or serious heart disease detected in these individuals, and only one person had clinically detectable heart disease. It was concluded that prolongation of the P-R interval is not a risk factor for heart disease.