To assess the effect of demographic and neonatal risk factors and outcome at the last available assessment on the probability of full responders, postal responders (those who only responded to the mailed questionnaire), or nonresponders in a follow-up study of 19-year-old adolescents who were born as preterm infants. The 19-year follow-up program was part of a large ongoing collaborative study in The Netherlands on the long-term effect of prematurity and dysmaturity on various medical, psychological, and social parameters. In the original cohort, 1338 infants (94%) with a gestational age of < 32 weeks and/or a birth weight of < 1500 g were enrolled. Neonatal mortality was 23% (n = 312), and another 67 children had died between the ages of 28 days and 19 years, leaving 959 survivors (72% of the original cohort) for follow-up at the present assessment. To study the effect of nonresponse, we divided the 959 survivors into 3 groups: full responders (596 [62.1%]), postal responders (109 [11.4%]), and nonresponders (254 [26.5%]). In the 3 groups we compared demographic and neonatal data, as well as outcome at the last available assessment. The odds ratios (ORs) for male versus female for the probabilities of nonresponse and postal response were statistically significant: 2.7 (95% CI: 1.9-3.9) and 1.6 (95% CI: 1.0-2.5), respectively. The same holds for the ORs for non-Dutch versus Dutch and low versus high maternal education for nonresponse: 2.0 (95% CI: 1.3-3.2) and 3.7 (95% CI: 2.0-6.7), respectively. Special education and severe handicap showed a statistically significant influence on nonresponse (OR: 1.6; 95% CI: 1.1-2.4 and OR: 2.6; 95% CI: 1.3-5.2) and postal response (OR: 2.0; 95% CI: 1.2-3.3 and OR: 4.4; 95% CI: 2.0-9.9), respectively. At the age of 19 years, primary school and special education were found significantly more frequent in the postal responders than in the full-response group (20% and 21% vs 6% and 12%). The full responders, on the other hand, were higher educated than were the postal responders. In this follow-up study at the age of 19 years, boys, non-Dutch adolescents, and low maternal education were overrepresented in the nonresponse and postal-response groups. Nonresponse decreased the proportion of infants with adverse outcome in assessed children. To be able to present reliable results for the total group of survivors in long-term follow-up studies, the nonresponse bias needs to be quantified. Therefore, it is evident that more research using statistical methods such as imputation of missing data is needed.