The paper examines the possible reasons why there was a different weight gain pattern in two groups of sick neonates fed two different amino acid solutions during a prospective double-blind trial of otherwise identical total parenteral nutrition. During the 13-month study period, 14 neonates (eight less than 32 weeks gestation) received Vamin as their amino acid source, and 18 (eight less than 32 weeks gestation) received a new amino acid solution, Paedmin. The older group of neonates gained weight far better when fed Vamin (P less than 0.003), neonates of less than 32 weeks gestation gained weight better when fed Paedmin (P less than 0.004). These differences in weight gain were reflected in differences in plasma amino acid concentration in that the levels were lower in the groups gaining weight less well; and in urinary fractional excretion where the groups gaining weight less well had a markedly higher fractional excretion and total urinary loss of amino acids (P less than 0.001). It is suggested that these differences in amino acid handling of two different amino acid solutions may lead to differences in weight gain.