1. Center for Biostatistics in AIDS Research, Harvard School of Public Health, Boston, Massachusetts 02115-6017.
2. Pediatric, Adolescent and Maternal AIDS Branch, National Institute of Child Health and Development, National Institutes of Health, Bethesda, Maryland 20814.
3. University of Southern California School of Medicine, Los Angeles County Medical Center, Los Angeles, California 90033.
4. Brigham and Women's Hospital, Boston, Massachusetts 02115.