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A study of regional differences in prevalence of primary adult lactose malabsorption in Italy was conducted on 205 subjects. Their origin was determined by their grandparents' birthplace, 89 from northern, 65 from central, and 51 from southern areas of Italy. Lactose malabsorption was diagnosed with standard oral lactose tolerance test and blood glucose determinations. Lactose malabsorbers showed symptoms more frequently than absorbers after the test load of lactose (p less than 0.01) they also reported milk intolerance more frequently (p less than 0.01). Prevalence of lactose malabsorption is significantly lower in the central sample (19%) than in the northern (52%) and southern (41%) samples (p less than 0.01). This finding contrasts with the hypothesis of a continuous increase in frequency of lactose malabsorption from northern to southern Europe and is probably due to the complex genetic history of the Italian population.