BACKGROUND: The relationship of developmental disability rates with difficulty obtaining follow-up data is unclear. With this study, we aimed to determine if children who attended research follow-up assessments with more difficulty had more disability at school age, compared with those who attended with less difficulty, and to establish the relationship between follow-up and disability rates. METHODS: Two groups, comprising 219 consecutive survivors born at <28 weeks' gestation or at <1000 g birth weight in the state of Victoria, Australia, in 2005, and 218 term-born, normal birth weight controls were assessed at 8 years of age for neurodevelopmental disability (any of IQ <-1 SD, cerebral palsy, blindness, or deafness). Children were classified as either more or less difficult to get to attend by research nurses involved in the study. RESULTS: The follow-up rate was 87% for both groups. Overall, children who attended with more difficulty had higher rates of neurodevelopmental disability (42%; 19 of 45) than those who attended with less difficulty (20%; 66 of 328) (odds ratio: 3.09, 95% confidence interval: 1.58 to 6.01; P = .001). As the follow-up rate rose among the 3 individual hospitals involved in the assessments, so did the rate of neurodevelopmental disability (P = .025). CONCLUSIONS: Children who attend with more difficulty have higher rates of neurodevelopmental disability at school age than those who attend with less difficulty, and disability rates rise with higher follow-up rates. Rates of neurodevelopmental disability will be underestimated if researchers are not persistent enough to obtain high follow-up rates.