A combination of lactose and glucose-galactose tolerance tests was carried out on 83 patients to determine the prevalence of isolated lactase deficiency in the the various ethnic groups of Nigerians living in Ibadan. Lactase deficiency was found in 81% of all the subjects. This consisted of 40 out of 48 Yorubas (84%), 9 out of 15 Hausa/Fulani (60%), 9 out of 11 Ibos (82%), and all 9 Nigerians belonging to other ethnic groups. The results indicate that lactase deficiency may be widespread throughout Nigeria. It is proposed that the high incidence of lactase deficiency is not caused by an inherited genetic defect of enzyme production in the intestinal mucosa, but might be due to a lack of induction from poor milk intake after weaning.