For more than a decade of investigation, the clinical and pathologic features of kuru, a subacute, fatal, progressive degenerative disease of the central nervous system, have remained constant; the disease remains confined to the Fore linguistic group and the neighbors with whom they intermarry. Dramatic changes, however, have taken place in the sex and age incidence of the disease, which has nearly disappeared among children. This has probably been the direct result of socioenvironmental changes taking place since 1957 in the kuru region. In the laboratory, the disease has been transmitted from a number of cases to the chimpanzee after an incubation period of 2 years. The simplest hypothesis that accommodates these findings, without stretching the other known factors, is that a conditionally infectious agent (slow virus), which demands the proper genetic and socioenvironmental background for transmission, causes the disease. The sociological phenomenon best able to explain the sex and age distribution of kuru, its historical spread, and its more recent changing pattern, is cannibalism.