BACKGROUND: Experimental data both supporting and refuting efficient lactose hydrolysis in enterally fed preterm babies is generally derived from fetal studies or using indirect assessments. Measurement of disaccharidase activity in the duodenal fluid of surviving preterm infants is a more direct approach. SUBJECTS: 63 infants, median (range) gestation 30 weeks (24 - 40) and birthweight 1300g (600 -2480).METHODS: Duodenal aspirates were obtained over the first 2 months of life. Disaccharidase activity was determined colorimetrically .RESULTS: 71/92 (77%) of lactase, 52/94 (55%) of sucrase and 59/93(63%) of maltase activities fell below the lower published reference range for older children. There was no increase in enzyme activities with increasing post-conceptual or post-natal age. No infant showed clinical evidence of malabsorption. CONCLUSIONS: Disaccharidase activities are frequently low in the preterm enterally fed newborn compared with older children. However, the absence of clinical evidence of lactose malabsorption suggests that lactose is assimilated in the colon. Preterm milk formulae need not contain less lactose than breast milk.