Mortality and incidence, complexity, and severity of early childhood neurodevelopmental disability are reported for two cohorts of preterm infants of 500 through 1250 g birth weight. Comparing 1978-1979 (cohort 1) and 1988-1989 (cohort 2), 1-year survival improved from 82 (36%) of 226 to 197 (67%) of 291. Cohort 1 survivors were heavier and more mature than cohort 2 survivors (1047 g vs 930 g, 29.6 vs 27.3 weeks). Parental demographic variables were similar. The incidence of specific disabilities with greater than 97% follow-up to 1.5 years adjusted age did not change: cerebral palsy, 14 (17%) vs 20 (10%); vision loss, 5 (6%) vs 9 (5%); mental retardation, 9 (11%) vs 13 (7%); hearing loss, 3 (4%) vs 7 (4%); and convulsive disorders, 2 (2%) vs 3 (2%). The overall number of disabled children (17 [21%] vs 30 [15%]), complexity of disability (> or = 2 disabilities per child: 11 [13%] vs 10 [5%]), and severity of disability (projected dependency: 6 [7%] vs 10 [5%]) did not differ between cohorts 1 and 2. The cerebral palsy prevalence, based on neonatal survival, dropped from 157 per 1000 to 93 per 1000. Analysis by birth weight-specific categories in 250-g increments did not alter results, but disability rates were highest for those of lowest weight. In contrast to other reports this population-based North American study from a well-developed perinatal regional program reports no increase in incidence, complexity, or severity of disability in preterm infants weighing 500 through 1250 g at birth.