Hydroxyprolinuria has been measured in 378 normal Zairean children aged between 3 and 13 years using the Kivirikko's method; 192 of them were boys and 186 were girls. Results show an age-related increase of urinary excretion of hydroxyproline in both sexes, girls having higher absolute values than boys. However, this difference is pronounced when values are corrected according to body size. Our values are slightly higher than those found in the literature.