1. From the Division of Hematology and Oncology. Children's Hospital, the Dana–Farber Cancer Institute, and the Department of Pediatrics, Harvard Medical School. Boston; the Cooperative Study of Sickle Cell Disease Coordinating Center, School of Public Health. University of Illinois. Chicago: and the Eastman School of Dentistry, Rochester, N.Y. Address reprint requests to Dr. Platt at the Division of Hematology and Oncology, Children's Hospital, 320 Longwood Ave., Boston, MA 02115.