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To evaluate the predictive value of blood pressures for future levels, longitudinal measurements were analyzed among Welsh subjects from age 5 to 74 at entry. The measurements were taken on 863 individuals from the Vale of Glamorgan in 1956, 1960, 1964, and 1971 and on 734 individuals from the Rhondda Fach in 1954, 1958, 1964, and 1971. The tracking correlation, defined as the correlation between blood pressure readings on the same person taken at two different times, is used as a descriptive measure of the magnitude of the association. The tracking correlations range from 0.25 to 0.6-0.7 with most of the increases occurring before age 20. The tracking correlation depends on initial age, sex, and time interval between measurements. The age-specific tracking correlations decrease as time interval between measurements increases. For a given time interval, the age-specific correlations are slightly higher for females than for males. These findings are very similar for each of the two regions studied.

The current conflict of views about the nature of essential hypertension reflects our ignorance of arterial pressure in the general population. Though reliable apparatus for the indirect measurement of bloodpressure has been available for over half a century we still do not know how arterial pressure increases with age in the population at large; we know little of the influence of environmental factors and there is disagreement about the mode of inheritance of hypertension. Investigations of these questions cannot easily be initiated from hospital practice, and it seems highly probable that the long-term study of representative populations by the modern techniques of epidemiology will contribute much to the understanding of the factors affecting blood-pressure within the next decade.Two such long-term epidemiological studies are in progress in South Wales, and this report summarizes the analyses of the genetic data derived from them. Other papers will deal with the influence of environmental and personal factors. The results reported here include data derived from the original " cross-sectional " surveys, and from the follow-up surveys which were undertaken four years later.Some of these data have been published previously (Miall and Oldham, 1955, 1957, 1958 Miall, 1962), but we believe that their re-examination in the light of subsequent analyses helps to form a clearer picture of the pattern of inheritance which is emerging.The Population Samples These surveys arose from the obvious need to investigate quantitatively the pattern of inheritance of arterial pressure in families that were not selected, as they had hitherto been, on the basis of propositi with and without hypertension defined in terms of some threshold pressure determining the boundary between normal and abnormal. The propositi were randomly selected from subjects over the age of 5 years in two geographically defined populations in South Wales. The populations and the methods of selection of random samples from them have been described elsewhere (Miall and Oldham, 1958). In the Rhondda Fach, 250 randomly selected propositi and 978 of their first-degree relatives cooperated in our surveys; in the Vale of Glamorgan a total of 373 propositi and 1,267 first-degree relatives cooperated. Over 95% of both the propositi and relatives living within 25 miles were examined in each survey. The completeness of the follow-up of these populations is shown in Table I. In the follow-up surveys over 99% of the Rhondda subjects and over 98% of those in the Vale were re-examined. Use of Age-adjusted ScoresThe hypothesis that essential hypertension is one manifestation of the inheritance of a single gene needs more detailed specification before it can be explored using data such as those now being presented. In particular, the hypothesis must specify how essential hypertension is defined in relation to arterial pressure at different ages in the two sexes. So far the proponents of the single-gene hypothesis have not suggested how this should be done, but have searched...

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