The aim of this study was to evaluate the prevalence of lactose malabsorption in a population of 75 (43 males, 32 females) apparently healthy school-children using the H2 breath test with cow's milk. The children, ranging in age from 8 years and 6 months to 15 years and 2 months (mean: 11 years, 7 months) were divided into 2 age groups: Group I (no. = 26): age < 11 years and Group II (no. = 49): age > 11 years. After on overnight fasting, lasting at least 8 hours, samples of expired air were collected at 0 time and at 30-min intervals following the administration of 250 ml cow's milk for a total time of 3 hours. The H2 breath concentration was then measured by gas chromatography (Micro-Lyzer Quintron Instruments Company mod. 12). A net rise of more than 20 ppm H2 was considered as lactose malabsorption. Subjects with symptoms such as excessive flatulence, abdominal pain, or diarrhoea, were considered as lactose intolerant. Two-hundred and 50 ml of cow's milk (12 g of lactose) was considered a more physiological vehicle than the traditional lactose aqueous solution. The examined children, all on free diet, showed a fasting alveolar from 0 to 43 ppm (mean +/- SD = 7.9 +/- 7.6). Sixty-six children out of 70 (93.3%), who completed the test had a total absorption of lactose. Two out of 4 children with lactose malabsorption originated from areas (East Africa and Central America), where a high incidence of this metabolic disorder is a characteristic findings.(ABSTRACT TRUNCATED AT 250 WORDS)