1. The total hydroxyproline:creatinine ratios in random urines collected every 3 months for 1 year from twenty-six adolescent boys (11–14 years) have been compared with the observed height and weight velocities of the boys during that year.
2. There were significant correlations of both height and weight velocities over 6 and 12 month periods with the ratios in the random urines collected during the same period.
3. The ratio in most boys fell from July to April and then rose during the final 3 months of the study. Sequential changes in the ratio correlated significantly with sequential changes in growth velocity (i.e. acceleration or deceleration) but the changes in the ratio occurring throughout the year could not be wholly explained by variation in growth rate.
4. The urinary total hydroxyproline:creatinine ratio is probably most useful as an index of growth rate in comparisons between children studied over the same period of time, or in sequential studies of the same child over a short period. In long-term sequential studies allowances must be made for factors other than growth velocity so that interpretation is more difficult.