Objectives: To investigate if the amount of peritoneal fluid (PF) in the Pouch of Douglas at oocyte pick-up (OPU) or OPU + 5 days predict severe late-onset ovarian hyperstimulation syndrome (OHSS) in women undergoing ovarian stimulation for assisted reproductive technology (ART). Study design: A secondary analysis of a dual-centre RCT on 1050 women referred for their first ART treatment in two public fertility clinics in Denmark and randomized 1:1 to GnRH-antagonist or GnRH-agonist protocol. All women from the two arms who were examined on day of OPU and OPU + 5 days were included in this study (n = 940). The ability of PF in the pouch of Douglas to predict severe late-onset OHSS was assessed by multivariate logistic regression analyses and receiver operator characteristics (ROC) curve analyses and compared with other known predictors of OHSS. The final models were cross-validated by the leave-one-out method to assess the models’ generalizability. Results: A total of 28 (3%) women developed severe late-onset OHSS. PF in the pouch of Douglas measured on OPU + 5 days predicted severe late-onset OHSS. The optimal cut-off value was 17.5 mm at OPU + 5 days with a 61% sensitivity and 71% specificity (Area under the curve = 0.70 95% CI 0.61–0.80). PF on the day of OPU was not predictive of late on-set OHSS as the adjusted multivariate logistic regression analyses showed insignificant results. Conclusion: Although PF in the pouch of Douglas could predict late-onset severe OHSS, the low sensitivity underlines that it is not useful as a sole marker to decide whether to perform blastocyst transfer or to use a freeze-all strategy.