The ovarian hyperstimulation syndrome (OHSS) is a serious but rarely fatal complication of medical interventions that seek to induce fertility; it is typically encountered in women who undergo controlled ovarian hyperstimulation, but there are very rare patients who have genetic defects who present with OHSS. In recent years, its pathogenesis has been elucidated; this knowledge will decrease the frequency of this syndrome. Clinical signs may include any or all of the following: rapid weight gain, ascites, oliguria, hemoconcentration, leukocytosis, along with intravascular hypovolemia, hyponatremia, and hyperkalemia. If the patient is not diagnosed early, ascites, pleural and more rarely pericardial effusions, severe respiratory failure and ARDS, hypercoagulability with tromboembolism and multiple organ system failure can occur. Due to the increased use of therapeutic strategies for infertility (particularly those using human chorionic gonadotropin), the systemic, particularly the pulmonary, complications of this syndrome must be identified early to allow appropriate diagnosis and management. We describe two cases of women with extremely severe OHSS presenting bilateral pleural effusions, and severe respiratory failure (paO2/FiO2 < 200) treated with non-invasive ventilation (NIV). The severe form of OHSS varies between 0.5% and 5%, depending on the population studied: intensive care may be required for management of tromboembolic complications, renal failure and severe respiratory failure. The diagnosis of severe OHSS was made, largely based on bedside ultrasonography showing increased ovarian size, mild ascites and bilateral pleural effusions. Owing to severe respiratory failure the patients were admitted to Respiratory Intermediate Care. Pulmonary intensive care may involve thoracentesis, oxygen supplementation and in more severe cases assisted ventilation. There are only a few studies in English that describe severe respiratory failure treated with non-invasive ventilation, but all of them have had good outcomes.