1. From the Division of Epidemiology (A.R.F., M.R.), School of Public Health, University of Minnesota, Minneapolis; the Division of Hematology (K.K.W., N.A.), University of Texas Medical School, Houston; the Collaborative Studies Coordinating Center (L.E.C.), Chapel Hill, NC; and the School of Hygiene and Public Health (F.J.N.), Johns Hopkins University, Baltimore, Md.