Thirty-three Mexican Americans between the ages of 9 and 60 were interviewed and tested for lactose intolerance. The participants of the study included 16 children and 17 persons not related by birth, including the parents of the children. Determination of lactose intolerance was based on a rise of less than 25mg/100 ml of blood glucose as measured by an Ames Dextrostix/Reflectance Meter following consumption of a lactose load. Forty-seven percent of the 17 nonrelated Mexican Americans were lactose intolerant. There was a marked relationship between low rise in blood glucose and symptoms of diarrhea, flatulence, and distention. Sixteen children from four families had an incidence of 50 per cent intolerance. The findings of intolerance in two successive generations of three families and in both sexes of the families adds support to the contention that lactose intolerance has a genetic basis, without sex predilection.