A brief, economic neonatal intervention based on the transactional model of development and influenced predominantly by the conceptual design of the Neonatal Behavioral Assessment Scale was implemented in an intensive care nursery with the mothers of a group of low-birthweight infants. The development of the intervention group was compared with that of a similar group of low-birthweight infants who did not receive the intervention and contrasted with that of a group of normal-birthweight infants. The intervention had a significant effect on maternal adjustment and perception of the infant at 6 months. No significant effect on infant cognitive development was apparent until 36 months (that is, 31 months after the intervention had ceased). The intervention effect was even more significant at 48 months. It appeared that the two low-birthweight groups had progressively diverged after 12 months, the intervention group rising until it approximated the normal-birthweight group in cognitive development, whereas the low-birthweight control group deteriorated. The economical nature of the MITP, its unique (although delayed) benefits, and the apparent durability of the intervention effect, suggest that this intervention program has important theoretical and practical implications and potentially far-reaching applications.