The present study investigated how neonatal behavioural organization was related to later mental development during the first year of life. The study further investigated the relationship between early social affective behaviour and later mental development. A group of 40 infants was assessed five times during the infants' first year, at three and 14 days and at four, eight and 12 months. The Brazelton Neonatal Behavioural Assessment Scale (NBAS), the Duve method (a clinically based method for observation of social affective behaviour), and Griffiths' Mental Development Scales were used to assess the development of the infants. The results indicate that early predictive behavioural components can be identified. Noenatal state control capacity was related to motor development at both eight and 12 months. Social responsivity at four months was related to hearing and speech at eight as well as at 12 months. The NBAS dimensions were good predictors of eight months performance, but were poor predictors of 12 months performance.