Objective. To assess the association between peak total serum bilirubin (PSB) levels during the first 2 weeks of life and neurodevelopmental outcomes of extremely low birth weight (ELBW) infants at 18 to 22 months' postmenstrual age. Methods. A retrospective analysis was conducted of a cohort of ELBW infants (401-1000 g) who survived to 14 days of age in the 12 participating centers of the National Institute of Child Health and Human Development Neonatal Research Network between January 1, 1994, and December 31, 1997. Demographic and clinical risk factors and PSB levels during the first 14 days were analyzed with reference to death or adverse neurodevelopmental outcomes at 18 to 22 months' postmenstrual age. The neurodevelopmental variables considered were Psychomotor Developmental Index (PDI) <70, Mental Developmental Index (MDI) <70, moderate or severe cerebral palsy (CP), hearing impairment (needs hearing aids), and a composite category designated as neurodevelopmental impairment (NDI). The NDI is defined as infants with any 1 or more of the following: PDI <70, MDI <70, moderate to severe CP, bilateral blindness, or bilateral hearing impairment requiring amplification. Results. The subjects of this cohort analysis are infants who were admitted to the Network centers during calendar years 1994-1997 and survived beyond 14 days and had PSB recorded during the 14-day period. From this cohort, 3246 infants survived at discharge, 79 died after discharge, and 592 were lost to follow-up. Thus, 2575 of 3167 infants were seen in the follow-up clinics with a compliance rate of 81%. Logistic regression analysis showed that various demographic and clinical variables are associated with poor neurodevelopmental outcomes. After adjustment for these risk factor, significant association were found between PSB (mg/dL) and death or NDI (odds ratio: 1.068; 95% confidence interval [CI]: 1.03-1.11); PDI <70 (R = 1.057; 95% CI: 1.00-1.12), and hearing impairment requiring hearing aids (odds ratio: 1138; 95% CI: 1.00-1.30). There was no significant association between PSB (mg/dL) and CP, MDI <70, and NDI. Conclusions. PSB concentrations during the first 2 weeks of life are directly correlated with death or NDI, hearing impairment, and PDI <70 in ELBW infants. The statistical association based on retrospective analysis of observational data and relatively small effect size should be interpreted with caution. Furthermore, because of the possibility of compounding effects of variables on outcome, the potential benefits of moderate hyperbilirubinemia and the potential adverse effects of phototherapy, a randomized, controlled trial of aggressive and conservative phototherapy is needed to address this controversial issue.