On the basis of original investigations on zinc, copper, and selenium levels in plasma and erythrocytes of Down's syndrome (DS), cystic fibrosis (CF), and control subjects, the possible importance of erythrocytic trace element concentrations in clinical analysis is emphasized. Red blood cell levels of copper and zinc were found significantly increased in both groups of diseased patients as compared to age-matched controls, although plasma levels did not statistically differ. Plasma selenium levels were significantly lower in both investigated groups, but red blood cell levels were only decreased in CF and were not different from controls in DS. Significant differences were also found between zinc, copper, and selenium levels in erythrocytes of two control groups originating from distinct geographic areas, although plasma levels were not statistically different. Some factors likely to modify trace element concentrations in erythrocytes are examined and a more systematic determination of these levels is suggested for use in clinical analysis.