1. Division of Immunologic and Infectious Diseases, Department of Pediatrics, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104
2. Westat, Rockville, Maryland 20850
3. Health Risk Reduction Projects, UCLA Research Center, Los Angeles, California 90025
4. Division of Adolescent Medicine, University of California at San Francisco, San Francisco, California 94143
5. Departments of Epidemiology and International Health, Medicine, and Pediatrics, University of Alabama at Birmingham, Birmingham, Alabama 35294