IN THIS investigation a single defect in childhood is examined with respect to a variety of factors which may be subtly associated with it. The abnormality, loss of distant visual acuity, is common and easily measured. Because the eye is so highly specialized. minimal changes in its structure produce disordered function that would go undetected in less specialized tissue. The study of an ocular disability, then, may reveal environmental and genetic influences that are not recognizable by current measurements of other organ systems. Unfortunately, loss of distant visual acuity is not a diagnosis but a physical sign produced by an array of eye diseases. An attempt will be made to sort these out and to focus attention on myopia, a defect with a pathogenesis that has intrigued physicians for centuries. BACKGROUND The study. In the years following the war pregnant women in Japan were allowed an extra ration of rice for which they could register beginning in the fifth month of pregnancy. This gave the Atomic Bomb Casualty Commission (ABCC) a case-finding mechanism that brought into its Genetics Program 93 per cent of all children born in Hiroshima and Nagasaki from mid-1948 through 1953. A full description of the plan and the outcome of 71,270 pregnancies is given in a monograph by NEEL and &HULL [I]. From the histories obtained it was found that about 5 per cent of the marriages were between first cousins and another 2 per cent were between first-cousins-once-removed *This work is a portion of a Child Health Survey sponsored by the U.S. Atomic Energy Commission, the Rockefeller Foundation, the Association for Aid to Crippled Children, and the U.S. Public Health Service’s National Institute of Dental Research. The Atomic Bomb Casualty Commission, a research agency of the National Academy of Sciences-National Research Council and the Japanese National Institute of Health, contributed facilities and personnel in Hiroshima and Nagasaki.