PURPOSE OF THE STUDY: Ovarian hyperstimulation syndrome is a potentially life-threatening complication during controlled ovarian stimulation for fertility treatment. Since no association of this condition with ABO blood groups was known, we compared ABO antigens with severity and onset of symptoms in a case-control study. PATIENTS AND METHODS: One hundred and twenty-one patients, mainly Caucasians, were hospitalized because of ovarian hyperstimulation syndrome after receiving in vitro fertilisation, in the period from January 2000 to February 2007. Severity of symptoms, pregnancy rate and ABO blood group were collated. The ABO blood group distribution was compared to four independent control groups. RESULTS: Blood group A was markedly more frequent and blood group O less frequent in patients with ovarian hyperstimulation syndrome compared to the blood group distribution in all control cohorts. The odds ratio for patients undergoing controlled ovarian stimulation with blood group A versus O to develop the early-onset form of this condition was 2.171 (p-value 0.002). No association for late-onset form could be found. The overall pregnancy rate was 50.4% and three times higher in the group of late-onset ovarian hyperstimulation syndrome compared to the early-onset form. Four patients developed thromboses in the jugular or subclavian vein, none of whom had blood group O. CONCLUSION: Blood group A may be associated with early-onset ovarian hyperstimulation syndrome in Caucasians. Depending on further studies, this possible association may be considered for an individualized hormone dosing in controlled ovarian stimulation.