A five-week-old Finnish boy was referred to the paediatric outpatient clinic of Turku University Hospital because of poor weight gain and failure-to-thrive. He was born at term after an uneventful pregnancy as the second child of healthy parents with a birth weight of 3810 g. He received breast milk until two weeks of age, then he was put on a regular, adapted, infant formula (Tutteli
®, Valio Ltd, Finland) but had poor weight gain and watery diarrhoea from the first days of life. At the age of 5 weeks, his weight was 150 grams below the birth weight though he ate reasonably big amounts of infant formula (900 ml/d). He showed metabolic acidosis (base excess was -12.3, pH 7.28, HCO3 14.9) and plasma sodium was high 165 mmol/l. In hospital, the values were corrected within a day with iv infusions. His faecal elastase was found to be normal, but his stools contained high amounts of reducing sugars. Because CLD was suspected, nutrition with lactose-free whey-based, hydrolyzed formula (Pepti-Junior
®, Nutricia, Netherlands) was started. The diarrhoea ceased during the first day and his weight gain improved remarkably. In specimen taken by gastroduodenoscopy, the morphology of his upper gastrointestinal tract was normal, but a very low activity of lactase was found in the duodenal specimen (because of the small size of the specimen, the laboratory did not give any exact lactase value, but they could not detect any activity) [
11]. Later, at the age of 11 months lactose malabsorption was verified by a positive breath hydrogen test. Growth was normal on a lactose free, whey-hydrolysate formula (Pepti-Junior). After the age 12 of months he was put on a lactose-free regular milk and he showed no gastrointestinal symptoms.