To evaluate the possible effects of outpatient preoperative evaluation (OPE) for new surgical patients who will be inpatients, we conducted an observational study at a university hospital in The Netherlands. Various outcomes before and after the introduction of an OPE clinic were compared. The study population comprised all 21,553 elective adult inpatients operated on between January 1, 1997 and December 31, 1999. Cardiac surgery, obstetric and pediatric patients, and patients operated on in same-day surgery were excluded. The main outcome measures were surgical cases canceled for medical reasons, rate of same-day admissions (who were expected to increase), and length of hospital stay. After introduction of OPE, the rate of cancellations for medical reasons decreased from 2.0% to 0.9% (adjusted odds ratio 0.7, 95% CI, 0.5--0.9). The rate of same-day admissions increased from 5.3% before to 7.7% after OPE introduction (adjusted odds ratio 1.2, 95% CI, 1.01--1.39), and the total hospital length of stay (in days) significantly decreased by a factor of 0.92 (0.90--0.94), which was partly the result of a reduction in preoperative admission time. We concluded that, although smaller than anticipated, the use of OPE for potential inpatients leads to a significant reduction of cancelled cases and of length of admission. Further increase of these benefits from OPE requires changes in institutional policy, such as forcing surgical departments to increase their number of same-day admissions. An observational study was conducted to compare various outcomes before and after the introduction of outpatient preoperative evaluation (OPE). Although smaller than anticipated, OPE for potential inpatients leads to a significant reduction of canceled cases and of length of admission.