In 42 low-birth-weight infants (smaller than 1,200 gm), we have compared the effects of intravenous nutrition supplement versus conventional feedings on growth, morbidity, mortality, and plasma amino acid patterns. Despite similar total caloric intake in INS and control groups, weight gain was greater in the INS infants. The overall mortality rate did not differ in the two groups. Nonsurviving infants receiving INS lived longer (mean equal to 30 days) than nonsurviving CON infants (mean equal to 5 days). Complications were equally frequent in both groups except that hyperglycemia occurred more often in infants receiving INS. The plasma aminogram of the LBW infant is described and compared to those of the full-term infant and adult. Hypoaminoacidemia was present at birth in the LBW infants, concentrations of glutamine, alamine, glycine, histidine, and ornithine being significantly (P smaller than 0.05) below FT values. During INS, elevations of threonine, serine, and methionine above FT values occurred. Glutamine remained subnormal despite INS. Recommendations for an INS solution more suitable for use in LBW infants are presented.