PIP: An investigation was conducted to compare duration of lactation in mothers giving birth to preterm infants vis-a-vis mothers giving birth to healthy full-term infants. A written questionnaire was mailed to all Norwegian women whose preterm infants were admitted to the Neonatal Unit, Department of Pediatrics, Ulleval Hospital over the 1978-80 period. The same questionnaire was mailed to an equal number of Norwegian women who gave birth to healthy full-term infants at the Ulleval Hospital. The control group consisted of the 1st mothers with uncomplicated full-term vaginal deliveries on the same days as the preterm deliveries. There were 155 mothers in each group. There were 100 completed questionnaires from the preterm group and 108 from the full-term group. Significantly more women who gave birth at term were able to breastfeed their infants completely at discharge from the nursery, compared with women who gave birth to preterm infants, whereas more of the preterm mothers partly breastfed their infants. In the preterm group no correlation was found between duration of breastfeeding and the infants' birth weight or between duration of breastfeeding and gestational age. Although a higher potential for breastfeeding existed in the full-term group, a high percentage of the preterm mothers succeeded in breastfeeding. Of the preterm infants, 61% were breastfed (wholly or in part) for 3 months, 45% for 6 months, and 21% for 9 months. The corresponding figures for full-term infants were 83%, 59%, and 38%. The figures for duration of breastfeeding among mothers giving birth to healthy full-term infants in the present investigation were significantly higher than in similar investigations performed during 1976-1977. A high potential for breastfeeding existed in women giving birth to preterm infants referred to the Neonatal Unit. This may be taken as biological evidence for human milk as a physiological basis for nutrition of preterm infants. The separation trauma after admission of a preterm infant to a neonatal care unit does not necessarily impair the mother's ability to breastfeed, and it may be more important that attitudes and practical routines in nurseries and neonatal care units support these women in sustaining lactation.