The likelihood of sustaining neurological, sensory or cognitive deficits is considerably greater for very low birthweight (VLBW) infants who require intensive care in early postnatal life than those without major neonatal illness. Identifying which, if any, medical events are responsible for an adverse outcome is most difficult in the face of multiple concurrent complications. In this research, a principal components analysis was performed in order to arrive at a set of orthogonal variables which succinctly described clinical involvement in the nursery. With this procedure, a single hypothetical factor depicting neonatal status (NS) was computed. Principal component scores were then generated for NS and assigned to 252 VLBW (less than 1500 g) infants. These subjects were followed prospectively from birth to 4 years of age. Standardized measures of neurological, sensory and intellectual function were regularly administered. Neonatal status was shown to be significantly correlated with the various test results and predictive of long-term development. When subjects were divided into quartiles with respect to NS, a specific subgroup was identified as "at high risk" for poor outcome. Those subjects falling into the lower quartile incurred more neurological abnormalities persisting beyond the first year. They also suffered a higher incidence of intracranial hemorrhage and sensori-neural hearing loss. In addition, the lower 25%, as a group, scored well below all others on traditional tests of mental ability. These differences were sustained throughout infancy and early childhood and could not be attributed to a number of demographic variables including sex, gestational age, birthweight, Apgar scores or parental educational level.