Methods: OHSS is quantified by consecutive measurements of weight, abdominal diameter, ultrasound measurements of ascites and ovarian volume, paraclinical parameters, and by a systematic patient questionnaire. Furthermore it is registered if the patients have been hospitalized and/or have had ascites drainage performed due to OHSS. Data from the first 100 patients are used together with Golans OHSS-classification, to make a more precise definition of OHSS. This definition is applied prospectively on data from the remaining patients.