For cases and controls, the following information was collected using a standardized data collection form: medical record number, date of abstraction, discharge date, date of birth, age (in years), sex, race, insurance type, chronic underlying condition(s) (yes or no), history of prematurity, allergies to medicines, patient hospitalization occurrence, immunization status (current or not current), medications used (yes or no), and a list of all current or recent medications recorded. For antibiotic use, if the medication was not listed as current, information on date of discontinuation before the date of ED registration was recorded, and an indication for treatment also was abstracted from the medical records. Generic and proprietary names of medications were identified and classified by one of us (D.M.S.) according to the National Drug Code Data Files of the Food and Drug Administration. Antibiotic indications were collected (otitis media, upper respiratory tract infection, urinary tract infection, and other), and "other" was defined if available from the ED record. Gastrointestinal (GI) medications were defined as antiemetics, intestinal promotility agents, H2-receptor antagonists, antacids, and antiflatulents. Gastrointestinal indications were categorized as gastroesophageal reflux, diarrhea, emesis, or other. Gastrointestinal medications were defined as medications whose primary purpose is to modulate clinical effects regarding the GI system. For case patients only, the method of intussusception diagnosis was recorded (clinical suggestion, surgical, radiographic, or unknown), and the anatomic location of the intussusception was identified if known.