INTRODUCTION: Ovarian hyperstimulation syndrome (OHSS) is one of the major complications of assisted reproductive technology treatment. We assumed that it had declined in recent years owing to the options of new preventive strategies. The aim of the present study was to investigate the annual incidence of OHSS in Denmark in the course of a 17-year period. METHODS: This was a national register-based historical cohort study including all women with an OHSS diagnosis admitted to Danish hospitals between 2001 and 2017. Data included information on all OHSS diagnoses, duration of hospital stay, early pregnancy complications and other complications like thromboembolism and ovarian torsion. The annual number of initiated stimulated in vitro fertilisation (IVF)/intracytoplasmic sperm injection (ICSI) cycles was based on the annual reporting by the Danish Fertility Society. RESULTS: From 2001 to 2017, a total of 2,261 (1.2%) women with an OHSS admission were identified among 186,168 stimulated IVF/ICSI cycles. The annual incidence of OHSS varied from 0.9% to 1.2-1.4% with no overall change over time (p = 0.24. Early OHSS (defined as OHSS without a pregnancy in the cycle) was seen in 48.5% of the events, late OHSS (defined as OHSS with a pregnancy in the cycle) in 51.2% of cycles. Among all women with OHSS, 75% were hospitalised for more than 24 hours, with mean 4.3 and 6.2 days in hospital for early and late OHSS, respectively. CONCLUSIONS: The annual incidence of severe OHSS leading to a hospital admission remained stable for 17 years, which suggests that OHSS preventive actions like use of antagonist cycles, agonist triggering and freeze all should be better implemented in Denmark.