6. Mathewson, F.A.L. and Brereton, D.C.: Atrio-ventricular heart block; University of Manitoba Follow-up Study reports - Series 1963: Trans Assoc Life Insur Med Dir of America 48: 210-234, 1964. Since the paper in 1952 additional members of the Manitoba Follow-up Study have been found to have a prolonged P-R interval for a total of 95 cases followed from 1948-1963. Fairly marked variation in P-R interval was found indicating changes in conduction through the AV node. There is a suggestion that the development of AV nodal conduction abnormalities is more significant in the older person, but as yet no evidence exists that individuals with prolongation of atrial ventricular conduction have an increase in morbidity or mortality due to heart disease.