We conducted a case-control study to identify clinical and demographic risk factors for admission to the hospital following ambulatory surgery. Of 9616 adult patients who underwent ambulatory surgery at a university-affiliated hospital between 1984 and 1986, one hundred were admitted. The most common reasons for admission were pain (18), excessive bleeding (18), and intractable vomiting (17). The mean age (±SD) of patients who were admitted was 37±13 years, and 96% had American Society of Anesthesiologists' physical status scores of 1 or 2. Factors that were independently associated with an increased likelihood of admission were general anesthesia (odds ratio, 5.2), postoperative emesis (odds ratio, 3.0), lower abdominal and urologic surgery (odds ratio, 2.9), time in the operating room greater than 1 hour (odds ratio, 2.7), and age (odds ratio, 2.6). Our results indicate that the likelihood of unanticipated admission is related more to the type of anesthesia and surgical procedure rather than to the patient's clinical characteristics.