Objective: to determine the growth potential of ambulatory surgery in the Netherlands. Setting: St. Antonius Hospital, Nieuwegein and SIG Health Care Information, Utrecht. Design: descriptive. Methods: numbers of admissions in the period 1984-1995 were obtained from Dutch bases of the National Hospital Institution (NZi). From SIG Health Care Information numbers were obtained with regard to seven specified interventions in the years 1991-1995, i.e. breast tumour excision, inguinal hernia repair, varicose vein operation, laparoscopic sterilisation, knee arthroscopy, cataract operation and tonsillectomy. The potential increase if any of the number of interventions in day care was determined by placing the hospitals in order of decreasing proportions of day care, and subsequently applying the proportions of the 5th and 10th hospitals, respectively, to the whole group. Results: the number of day care admissions rose from 172 000 (9.9% of all admissions) to 649 000 (29.1%). Of all interventions studied, the percentage carried out in day care increased; the percentages varied greatly within hospitals as well as from one hospital to another. In 1995, the mean number of the seven interventions in day surgery was 115 000 (57% of all 201 000 interventions). The shift from interventions during hospitalisation to day care would be 42 000 and 51 000 (21 and 25%, respectively, of 201 000), respectively; operations performed in day care would then amount to 166 000 (83% of the total number of interventions) and 157 000 (78%). Conclusion: of the interventions studied, the proportion carried out in day care increased to 57%. In view of the interhospital differences, a considerable increase of day care is possible in the near future.