Growth (weight, length, mid-upper arm circumference), acid-base status, serum electrolyte levels, and selected parameters of urine (creatinine and electrolyte levels, urine pH, renal net acid excretion, levels of metabolites of aldosterone) were determined serially from the 10th to the 32nd days for 4 weeks in 21 premature infants (birth weight, 1,100-2,000 g) fed either formula A or formula B (formula B was formula A supplemented with chloride, potassium, and calcium). Premature infants fed formula B showed a higher weight gain (31 versus 28.2 g/day), a higher increment of middle-upper arm circumference (0.31 versus 0.24 cm/week), and a decreased renal net acid excretion (1.24 versus 1.92 mEq/kg/day). Almost all premature infants fed formula A and some with a high growth rate receiving formula B showed hypochloruria corresponding to chloride deficiency. Premature infants fed unsupplemented humanized formulas may have an inadequate intake of minerals.