To assess whether nonorganic nutritional dwarfing (ND) is accompanied by a biochemical adaptation to reduced nutrient intake, Na+,K(+)-ATPase activity of erythrocytes was examined. The study included 27 children with ND who demonstrated deteriorating linear growth and poor weight gain (caused by self-imposed dietary restrictions), 20 patients who initially exhibited ND growth patterns and subsequently experienced catch-up weight gain and growth after nutritional rehabilitation (NDR), and 32 normally growing children who had familial short stature and/or constitutional growth delay (F/CSS). Na+,K(+)-ATPase activity in erythrocytes was significantly lower in ND patients than in the NDR and F/CSS groups. Furthermore, Na+,K(+)-ATPase activity was positively correlated with incremental body weight gain. Na+,K(+)-ATPase concentrations did not differ significantly with regard to sex, chronological age, bone age, or pubertal status. These data suggest that the growth retardation of ND patients is associated with decreased erythrocyte Na+,K(+)-ATPase activity without other biochemical evidence of malnutrition.