Objectives. The purpose of this study was to investigate possible bias in the evaluation of neurodevelopment and somatic growth at 18 to 22 months' postmenstrual age among extremely low birth weight (ELBW) survivors (401-1000 g at birth). Methods. Data from a cohort of 1483 ELBW infant survivors who were born January 1993 through December 1994 and cared for at centers in the Neonatal Research Network of the National Institute of Child Health and Human Development were examined retrospectively. Children who were compliant with an 18- to 22-month follow-up visit, who visited but were not measured, or who made no visit were compared regarding 4 outcomes: 1) Bayley Scales of Infant Development, 2nd edition, Mental Developmental Index (MDI) <70 and 2) Psychomotor Developmental Index (PDI) <70, 3) presence or absence of cerebral palsy, and 4) weight <10th percentile for age. Logistic regression models were used to predict likelihood of these outcomes for children with no follow-up evaluation, and predicted probability distributions were compared across the groups. Results. Compared with children who were lost to follow-up, those who were compliant with follow-up were more likely to have been 1 of a multiple birth, to have received postnatal glucocorticoids, and to have had chronic lung disease. These factors were significantly associated with MDI and PDI <70 in the compliant group. Chronic lung disease was associated with increased risk of cerebral palsy (CP). MDI and PDI scores <70 were found in 37% and 29% of children who were evaluated at follow-up, respectively. Prediction models revealed that 34% and 26% of infants in the no-visit group would have had MDI and PDI scores <70. Compliant children tended to have greater incidence of MDI <70 compared with those predicted in the no-visit group but not PDI <70. CP was identified in 17% of the compliant group and predicted for 18% of the no-visit group. Predicted probabilities of having CP were marginally higher among the no-visit infants compared with those who were compliant with follow-up. There were no statistically significant somatic growth differences among the compliant, visit but not measured, and no-visit groups. Conclusion. ELBW infant survivors who weighed 401 to 1000 g at birth and who are compliant with follow-up evaluations may have worse Bayley Scales of Infant Development, 2nd edition, MDI scores than infants with no visit. Thus, follow-up studies based on infants who are compliant with follow-up care may lead to an overestimation of adverse outcomes in ELBW survivors.