Isosmolar coma occurred during the first week of parenteral nutrition in four infants when the amount of protein hydrolysate infused was rapidly increased to or above 4 gm/kg/day. Coma developed within a 6- to 12-hour period and was characterized by (1) a normal serum osmolality, (2) an elevated urine specific gravity, and (3) the rapid reversal of the clinical signs when the protein containing solution was discontinued. A very gradual increase in protein content to a total of no more than 4 gm/kg/day is advised to provide an adequate yet safe protein load for intravenous nutrition.