A total of 308 healthy Italian adults (192 females, 116 males; mean age 29.2 yr) were examined using a field version of the lactose tolerance test with breath hydrogen determination. Two geographical groups were formed according to the birth places of the probands' grandparents: 208 subjects from northern Italy (mainly from the regions of Piemonte, Lombardia, and Veneto) and 100 probands from Sicily. Lactose malabsorption was diagnosed in 106 subjects in group "north" (51%) and in 71 subjects in group "Sicily" (71%). Awareness of milk intolerance was more frequent in lactose malabsorbers. The incidence of diarrhea after the test dose of lactose was significantly higher in "aware" lactose malabsorbers, when compared with persons of the same group who had not experienced milk intolerance. The significant difference in lactose malabsorption frequency between northern Italy and Sicily is further evidence of a north-south gradient of lactase gene frequencies in Europe.