Introduction: An isolated unilateral pleural effusion as the only presentation of ovarian hyperstimulation syndrome (OHSS) is very rare. This case is an unusual presentation of OHSS after a confirmed respiratory tract infection, with no other coexisting risk factors identified for this syndrome. We also imply that the presence of Haemophilus influenzae in bronchial fluid can increase local reaction to vasoactive cytokines. Case Report: A 32-year-old woman presented at the Department of Reproductive Medicine and Gynaecology of the Pomeranian Medical University after 10 years of infertility with diagnosed hyperprolactinemia followed by bromocriptine treatment. The patient had three IUIs but no pregnancy was achieved. Therefore, ICSI was proposed. After an ovarian hyperstimulation, oocyte aspiration gave 8 oocytes. Although ICSI was performed in all of the oocytes there were 3 fertilizations. The ET of 3 embryos was carried out following 3 days of culture. Three weeks before the gonadotropin administration and a week before GnRH administration the patient had a respiratory tract infection with the most typical syndromes. The infection was treated successfully with over-the-counter medications and antibiotic .Three days after ET the patient was admitted to the ICU with signs of severe dyspnoea. The chest Xray showed a large pleural effusion over the right lung. Upon admission, thoracocentesis was preformed and 1600 ml of clear fluid was aspirated. The bronchial aspirate showed evidence of Haemophilus influenzae and leukocytes. After three days of standard treatment the chest X-ray revealed no pathology. The patient was discharged asymptomatic on the 4th day of treatment. Serum beta-hCG level was negative on day 12 after ET. Conclusions: This case suggest that respiratory tract infection prior to stimulation may constitute a new independent risk factor for OHSS. However, the true relation between the respiratory tract infection and susceptibility to OHSS still awaits explanation. Recent or existing respiratory tract infection may be a relative contraindication for starting COH.