Lactase deficiency and lactose tolerance were studied in several Jewish communities in Israel. Lactase deficiency was found in 60% of biopsied subjects. Forty-one biopsies were peroral and 22, surgical; histologically, the jejunal mucosa of all specimens was normal. Lactose tolerance tests were performed on 217 subjects, 118 patients and 99normals. A low glucose rise was found in 44.4% of Yemenites, 62.5% of North Africans (Sephardi), 72.2% of others (Sephardi), 79.2% of Ashkenazi, 84.2% of Iraqis and 85.0% of others (Oriental); the overall incidence was 71.1%. Statistical analysis confirmed that the population studied was heterogenous. Most subjects with a low glucose rise had symptoms during the test. Milk intake, low in almost all subjects, did not correlate with lactose tolerance. Most lactose intolerant subjects were not aware of milk intolerance; the condition is not usually symptomatic in Israel. Lactase deficiency in various population groups has been reviewed and evidence in favor of a genetic etiology emphasized.