We assessed the efficacy of an early intervention program (EIP) designed to promote the neonatal behavioral organization and developmental progress in infants born prematurely. The study subjects consisted of 48 very low birthweight infants (< or = 1,500 gms) who received care in NICU of Nagasaki University Hospital, Nagasaki, Japan. During 1993-1996, the experimental group (n = 30) received the NBAS (Neonatal Behavioral Assessment Scale) based intervention combined with a therapeutic program (EIP), for 44 weeks starting from 38 weeks of postmenstrual age. The control group (historical control, n = 18) received the standard medical-nursing care without EIP during 1990-1992. Developmental outcome was assessed in both groups using the NBAS examination after EIP at 44 weeks of postmenstrual age, and the Bayley Scales at 12 months corrected age. Multivariate analyses were performed to adjust for baseline variables that might be associated with the developmental outcome: sex, appropriate or light for dates infant, birthweight, gestational age, Apgar score at 1 minute, duration of intubation, and NBAS cluster score before EIP. In the NBAS examination after EIP, the experimental group achieved significantly higher scores in orientation, motor performance, state range and state regulation tasks than the control group. Furthermore, the experimental group showed significantly higher Bayley Mental and Psychomotor Developmental Indices compared with the control group. These results suggested that EIP was effective in promoting the neonatal neurobehavioral development of very low birthweight infants.