This paper, in two parts, is intended to relate recent medical findings on differences in lactose tolerance among the world's peoples to research on the history of milking. In this section, Part I, the medical research is reviewed, the pattern of group differences in tolerance is sketched, and hypotheses are advanced for these differences and are then evaluated. The evidence supports a genetic etiology for the differences. It also demonstrates that the illness experienced by nonmilking individuals on drinking milk is not, as has often been assumed by scholars, simply of psychosomatic origin. Thus, the nonmilking attitude, a cluster of views that nonmilkers use to explain their abstention from milk, involves elements of biological as well as of cultural origin. In Part II of this paper, the distributional pattern of lactose-intolerant groups will be viewed against the history of milk use.