Plasma Cr concentrations have been studied in normal children aged 0-14 yr. Levels ranged from 0.65 to 0.88 microgram/l and did not change with age. Plasma concentrations of CF patients given 0.5-0.75 microgram Cr/kg/d in addition to their diet were similar to normal values. There was no correlation between these plasma values and growth retardation.