Lactose intolerance is a condition suffered by an estimated 50 million Americans. Certain ethnic and racial populations are more widely affected than others. As many as 75 percent of all African-American, Jewish, Native American, and Mexican-American adults, and 90 percent of Asian-American adults are lactose intolerant. Some populations in Africa are completely lactose intolerant, whereas some northern European populations are unaffected. Why so? Genetic evidence indicates that lactose intolerance arose over 5,000 years ago and spread among populations due to positive selection. Correlations have revealed lactose intolerance is prevalent in geographical areas of extreme climates and where there have been persistent occurrences of communicable diseases affecting cattle. (Contains 8 figures and 2 tables.)