A prospective survey of applicants for employment was undertaken to investigate the relationship between heart diameter and cardiothoracic ratio (measured in 100 mm chest radiographs) age and ethnic group, 1432 male applicants, aged 17-64 years, were studied, of whom 861 caucasians, 295 Asians and 172 African/West Indians were included in the detailed statistical analysis. A significant relationship was found between the cardiac measurements and age, which differed within ethnic groups. The median values of cardiothoracic ratio were: 43% in caucasians, 44% in Asians, and 46% in Africans. The upper extreme observations in each regression (2.5% of the population) are delimited by confidence limits plotted on the regressions against age for both cardiac diameter and cardiothoracic ratio. It is concluded that a single upper limit (e.g. 50%) for cardiothoracic ratio is unsatisfactory. If all subjects with values of cardiothoracic ratio greater than 50% in the present sample had been recalled for more detailed cardiological investigation, this would have affected 2.2% of caucasians, 4.1% of Asians, and 9.3% of Africans. Limits of 50% in caucasians, 52% in Asians and 53% in Africans would exclude 2.2, 2.4 and 2.6% of subjects in each of these racial groups in our sample. Age accounted for relatively little of the variation observed.

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