Four thousand three hundred and thirteen children beginning at five to fourteen years of age have been examined on three to six occasions in Muscatine, Iowa, on alternate years. To compare blood pressures throughout the period of observation each value was expressed as a percentile rank. For each subject the average percentile rank (level), the trend in rank and the variability over time were calculated. Values for height, weight, relative weight and triceps skinfold thickness measurements were expressed in the same fashion. There is a relationship between average rank of blood pressure and average rank of body size as well as between trend of blood pressure and trend of body size percentiles. These observations indicate the importance of relative rate of growth in the establishment of the rank order of blood pressure during childhood.