1. Gastrointestinal Unit, Massachusetts General Hospital, Boston, MA 02114.
2. Center for Biostatistics in AIDS Research, Harvard School of Public Health, Boston, MA 02115.
3. Division of Digestive Diseases, University of Cincinnati College of Medicine, Cincinnati, OH 45267.
4. Division of Infectious Diseases, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, MA 02215.
5. Division of Gastroenterology, University of California, San Francisco, CA 94143.