Twenty-six children with marasmus and 27 with kwashiorkor were compared with 23 control children of matching ages. Kwashiorkor was characterized by increased phospholipids (NS), low (P < 0.01) apolipoprotein (apo) B-rich LDL, and near normal apo A-I and HDL-C. In children with marasmus apo B (P < 0.02) LDL-C (NS), apo A-I (P < 0.01), and HDL-C (P < 0.001) decreased. Fifteen children in each group were followed for 2 wk. Control values were progressively reached after 2 wk. In the younger children final apo B was higher than in control subjects (P < 0.03) but apo A-I was identical. Apo A-IV, assayed because it correlates with the functional state of intestine, was near normal in children with kwashiorkor and decreased with treatment. In children with marasmus apo A-IV decreased by 50%, increased with treatment in older children, but further diminished in younger children. After 2 wk apo A-IV was significantly lower in all patients than in control subjects. Apo A-IV, by remaining depressed after other variables normalized, seems a good index of nutritional status.