Improving survival of extremely low birthweight (< 1000 g) preterm infants opens the practical issues of their postnatal nutrition and growth. The authors studied nutrition and weight gain in 16 extremely low birthweight preterm infants (birthweight: 890 +/- 22 g, gestational age: 28.0 +/- 0.2 week, mean +/- SEM) during the first 12 weeks of life. Milk of the mother, or fortified pooled human milk or preterm infant formula was fed. The preterm infants approximated their birthweight by the end of the 3rd week of life (21st day: 866 +/- 29 g). Body weight expressed as per cent of birthweight was 109 +/- 2% at the end of the 4th, 176 +/- 7% at the end of the 8th and 275 +/- 6% at the end of the 12th week of life. Weight gain during the 1st to 8th postnatal weeks was compared to the mean in utero weight gain of foetuses with identical gestational age, gender and weight percentile position. Cumulative weight gain of preterm infants during the first 8 weeks of life was significantly lower than that of the theoretical controls (76 +/- 7% versus 136 +/- 2%, per cent of the initial value, preterm versus control, p < 0.0001). Additional weight gain of preterm infants was lower than that of the controls on the 1st to 5th weeks of life (g/kg/day, 1st week: -14.4 +/- 1.6 versus 16.7 +/- 0.5, p < 0.0001; 5th week: 13.3 +/- 1.2 versus 16.4 +/- 0.3, p < 0.05), there were no differences between the two groups on the 6th and 7th weeks, whereas preterm infants gained significantly more weight on the 8th week of life than the theoretical control value (18.2 +/- 0.9 versus 14.0 +/- 0.2, p < 0.001). These data indicate that the first weeks of life represent an especially important period for the improvement of the nutrition of extremely low birthweight preterm infants.