The objectives of this study were to analyse the results of an ambulatory surgery unit and the influence of short stay hospitalisation (24 h) on its activity. Between May 1992 and January 1998, 12412 patients have been treated. The most active speciality was general surgery, with 5567 interventions: 3.756 were performed on an ambulatory basis, and 1811 with 24 h hospitalisation. The global substitution index for this speciality was 54.7% (78.7% if we include the patients admitted overnight). 24 h Hospitalisation favours an increment of the substitution indices by allowing more flexibility in the selection criteria and more complex cases to be treated.