Unexpected admission (UA) to the intensive care unit (ICU) among the ambulatory patient could be considered as an indicator of quality of attention on ambulatory services. However, the determinants associated with this rare complication are unknown. Objective. To identify the factors associated with UA to the ICU among the patients following ambulatory surgical procedures (ASP).Twenty one cases and 105 controls were selected from among 4,705 patients admitted for an ASP at a teaching hospital between January 2004 and May 2006. A case was that one with an UA to the ICU for monitoring and/or treatment after its ambulatory surgical procedure. Each case was paired with five controls according to type of procedure and date of accomplishment. Conditional logistic regression analyses were used to determine the potential factors associated with an UA to the ICU.Cases represented 0.4% of admitted ones for an ASP. Mean age of controls were 46.9 years and 52.4% were women. There were no statistically significant differences between cases and controls in relation to several clinical, biochemical and physical status variables (comorbidity, vital signs, biochemistry, surgical procedure, anesthetic, technical anesthetic, time of surgery, surgical bleeding). Fourteen patients were less than 48 hours at the ICU and there were no deaths.Most of UA to the ICU seem to be based on decisions non-related to general health status of patients and these are usually preventive. This decision is not based on scientific evidence. Admission to the ICU must be based on a multidisciplinary evaluation.