Hematocrits were studied in 471 children aged 11, 12, or 13 years who resided in an affluent suburb of Pittsburgh. It was found that different microhematocrit machines gave slightly different results, so all values were corrected to a reading that would have been obtained using a Wintrobe hematocrit tube. A true Gaussian curve was obtained, suggesting that these children are a valid source of "normal" values. Mean hematocrit was 43%, with 95% confidence limits of 39 to 47%, and was the same for boys and girls. These are slightly higher values than usually are accepted as normal for this age group, and for the girls, values are comparable to those of women. A group of 220 black children of the same age who participated in a sickle-cell screening program had comparable mean hematocrits, but the distribution curve was skewed, suggesting that a small but significant number were anemic.