Lactose tolerance tests were performed on 167 people above age 12 from 32 Jewish families in Israel. One hundred-five subjects (63%) with a maximal glucose rise of less than 20 mg per 100 ml during the lactose tolerance tests were considered to have primary adult lactase deficiency. Analysis of data in 30 parental couples and their 88 offspring showed a significantly (P < 0.01) nonrandom distribution of lactase deficiency among the offspring, depending upon the mating types of the parents. The results support the conclusion that lactase deficiency is a hereditary condition. The mode of inheritance remains uncertain.