Pulmonary hydatid cyst in pregnancy is a very rare pathology and its diagnosis and treatment is stilt a complex of problem. We report a rare case of ruptured giant pulmonary hydatid cyst presenting with tension pneumothorax during pregnancy. According to our knowledge this is the first report of such a case. A 21-year old pregnant woman was admitted to our hospital with complaints of left-sided chest pain, cyanosis and dyspnea. Chest radiograph showed tension pneumothorax, mediastinal shift, and tracheal displacement. Echocardiography revealed perforated hydatid cyst adjacent to pericardium. She was taken to the operating room immediately. During operation, a giant perforated hydatid cyst (12x10cm) was found, outside the pericardium displacing and compressing the left lower lobe. Histopathological examination confirmed the diagnosis. Approximately 5 months later she had a spontaneous vaginal delivery. Both the patient and her baby were healthy. Perforated pulmonary hydatid cyst should be kept in mind in the differential diagnosis of tension pneumothorax in a pregnant woman and surgical intervention should be performed promptly.