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The study consisted of 172 subjects belonging to ethnic groups from Sinai in the Eastern Desert and the New Valley in the Western Desert with respective mean ages of 36.7 +/- 2.0 and 26.6 +/- 1.0 years. Lactose absorption was assessed by measurement of urinary galactose in pooled 2-hr urine samples following ingestion of an oral lactose dose of 40 g. Mean 2-hr excretion values after ingestion were 32.3 mg galactose in the Sinai and 77 mg in the New Valley. In the evaluation of lactose malabsorption, a diagnosis of lactose malabsorption is based upon a cutoff point of 0.075 mg/mg urinary galactose:creatinine ratio. The overall prevalence rate in those populations is 34.3%. The proportion of lactose malabsorbers was 11.1% in Sinai and 51.0% in the New Valley. Highly significant differences (chi(2) = 29.5, P < 0.0001) were found between the two ethnic groups with regard to the frequency distribution of lactose malabsorption. The existence of an east-west gradient of increasing frequencies of lactose malabsorption gene is suggested.