We measured the plasma concentration of allantoin, an oxidation product of uric acid and an "in vivo" marker of free radical generation, within 24-48 h after birth in 10 premature infants who subsequently developed chronic lung disease (CLD) and 9 infants without CLD (non-CLD). The plasma allantoin level (mean +/- SD, 25.9 +/- 9.8 microM for CLD versus 11.0 +/- 5.7 microM for non-CLD, p < 0.01) and the allantoin/urate ratio (5.8 +/- 2.0% for CLD infants versus 2.4 +/- 0.9% for non-CLD infants, p < 0.01) were significantly higher in the CLD group than those in the non-CLD group. These observations suggest the possible involvement of oxygen radicals in triggering CLD. In addition, the plasma allantoin concentration and the allantoin/urate ratio may be useful early predictors of the development of CLD.