A B S T R A C T Intravenous hyperalimentation was done in 11 underweight adults whose body weight (body wt) was less than 85% of ideal. For the first 6 days, "complete formula" was infused furnishing per kilogram ideal body wt per day: 15 g glucose, 0.40 g N, 0.018 g P, 2.4 meq K, 3.0 meq Na, 2.3 meq Cl, 0.5 meq Mg, 0.45 meq Ca, and 50 ml H20. Patients gained weight at an average rate of 9.0 g/kg ideal body wt/day and showed average balances/kilogram ideal body wt/day as follows: + 0.14 g N; + 0.012 g P; + 0.43 meq K; + 0.49 meq Na; + 0.37 meq Cl; and + 0.085 meq Ca. Application of standard equations to the elemental balances indicated weight gain consisted of 35-50% protoplasm, 35-50% extracellular fluid, 5-25% adipose tissue, and < 1% bone.Withdrawal of N, P, Na, or K impaired or abolished retention of other elements. Removal of N halted retention of P, K, Na, and Cl; withdrawal of K stopped retention of N and P; and removal of Na or P interrupted retention of all other elements. Weight gain continued at a rate of 1.4-3