The effects of preterm birth and the perinatal infant health condition on mother-infant interactions were analysed in 278 mother-infant pairs, divided into four groups according to infants' gestational age at birth: group 1, 23-31 weeks; group 2, 32-36 weeks; group 3, 37-42 weeks; and group 4, a control group of healthy full-term infants. The methodological approach was based on observation of the pairs at 2, 4 and 6 months of infants' corrected age (+/- 1 week) during undressing of the infant and face-to-face interaction. It was found that mother-infant pairs with preterm infants (groups 1 and 2) did not differ in interactional variables from those of the control group. On the other hand, the birth of a full-term infant in need of neonatal intensive care (group 3) affected maternal and infant interactive behaviour. Additionally, infants from group 3 did not show stability in their interactive behaviour between any ages of measurement. This result suggests that interactive behaviour of full-term infants in need of neonatal intensive care are rather unpredictable during their first 6 months of life, which might have contributed to the less optimal interactive pattern observed for their mothers compared with mothers of the control group.