Absorption of dietary calcium was evaluated with the extrinsic tag approach and stable isotope methodology in growing premature infants. Fractional absorption of a bolus dose of 46Ca was determined on 16 occasions in 13 premature infants (birth weight 1135 +/- 40 g, gestational age 29.5 +/- 0.4 wk, mean +/- SE) and was found to be 84.4 +/- 2.2%. Fractional absorption of 46Ca ranged between 65 and 97%, and did not appear to be influenced by postnatal age, postconceptual age, body weight, or intake of preterm human milk, fortified preterm human milk, or premature formula. Therefore, if absorption of the 46Ca dose reflects that of dietary calcium, about 80% of dietary calcium is absorbed.