Analyses are reported on the correlation with height and with subcutaneous fat thickness of relative weight expressed as per cent of average weight at given height, and of the ratios weight/height, weight/height squared, and the ponderal index (cube root of weight divided by height) in 7424 ‘healthy’ men in 12 cohorts in five countries. Analyses are also reported on the relationship of those indicators of relative weight to body density in 180 young men and in 248 men aged 49–59. Judged by the criteria of correlation with height (lowest is best) and to measures of body fatness (highest is best), the ponderal index is the poorest of the relative weight indices studied. The ratio of weight to height squared, here termed the body mass index, is slightly better in these respects than the simple ratio of weight to height. The body mass index seems preferable over other indices of relative weight on these grounds as well as on the simplicity of the calculation and, in contrast to percentage of average weight, the applicability to all populations at all times.