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Recent studies have shown that lactose malabsorption (LM) due to lactase deficiency varies in frequency in different populations, from 2.6-6.6 56 in Danish gastroenterological patients (8) to 100% in healthy adult Thais (6). However, there are few investigations of the age at which lactase deficiency appears in different populations. Virtually all the Thai children older than 2-4 years who were studied had isolated L M (6, 13) and in Bantu tribes in Uganda LM seems to appear very early (5).Recently Bolin and his co-workers (4) showed that LM is universal above 10 years of age in Singapore.In an earlier study by our group the prevalence of LM in a Finnish rural population from 21 to 65 years of age was 15% (12). As the prevalence was similar in all age groups and congenital L M is extremely rare, 7-15year-old children from the same community were studied to discover the prevalence of L M in children of school age and the age at which LM appears. We also noted abdominal complaints and the effect of a lactose-free diet, to get a picture of the practical significance of LM. MATERIAL AND METHODSPornainen in Southern Finland is a small rural community, the main occupation in which is agriculture (about 60%). One-quarter of the inhabitants came as evacuees from Karelia during the Second World War.In 1969 the total number of 7-15-year-old children in Pornainen was 380, 162 of these were selected by simple random sampling. The families were sent a questionnaire concerning the children's abdominal complaints and food habits. The children were all invited to undergo an examination to detect 'functional disorders of the small intestine'. Of the children selected 11 had moved from the community, and of the remaining 151 children the questionnaires were returned by 150. Of these, 130 (60 boys and 70 girls) were willing to participate in ths examination, i.e. 80% of the 162 children selected. There was no statistically significant difference between those who participated and those who declined examination as regards age, sex, milk-drinking habits and occurrence of abdominal complaints. Lactose tolerance test (LTT)After overnight fasting, 1 g/kg of lactose was given as a 12.5% solution, the maximum dose being 50 g. Capillary b!ood samples were taken before and 20 and 40 min after lactose ingestion. The glucose concentration was determined by the glucose oxidase method (10). Abdominal symptoms on the day of LTT were noted. A maximum rise in blood glucose concentration of 25 mg/100 ml or more was taken as a sign of normal lactose absorption. The children who had a borderline rise between 20 and 24 mg/100 ml had a second LTT with double doses of lactose, 2 g / kg. Those with a maximum rise of 20 mg/100 ml or more in this repeated LTT were considered to have normal lactose absorption.Chiidren with maximum rises of less than 20 mgi 100 ml in the first or repeated LTT were sent for a small-intestinal biopsy to the outpatient department of the Children's Hospital, University Central Hospital of Helsinki. Biopsy samples were examined w...