Postoperative sinus cysts are known to be slow growing and it normally takes 15 to 25 years for them to become symptomatic. We report herein on an extraordinary case with frontal sinus cysts developing and ballooning intracranially 1 year after endoscopic sinus surgery. A 26-year-old male underwent bilateral endoscopic sinus surgery under the diagnosis of eosinophilic chronic sinusitis. The postoperative follow up CT in 5 months revealed recurrent polyps in the nasal cavity and thickened mucosa in every sinus, as a nature of eosinophilic chronic sinusitis. One year postoperatively, CT and MRI imaging showed that large bilateral frontal sinus cysts had eroded the posterior wall of the frontal sinuses, widely extended into the anterior cranial fossa and compressed the left frontal lobe. Interestingly, the patient did not complain of any symptoms, central nervous system deficits, headaches or head heaviness though the cysts developed in the rather short period of seven months. In order to widely open the cysts and to observe the condition of the frontal sinus, we employed a modified Lothrop procedure with small extranasal control holes. The content of the cysts was a serous and clear effusion and differed from the known content of postoperative cysts which is ordinary a dark color mucous with high viscosity. The patient has not demonstrated any relapse of his frontal sinus cysts for 1 year and 8 months after the surgery. Our case is an extremely rare condition of frontal sinus cysts because of the speed of their development, the extraordinary contents of the cysts and the lack of symptoms. To the best of our knowledge, and despite an extensive search, no other such a case has ever been reported in the literature. The mechanisms of the development of these cysts are unknown but supposed to be partially due to the cause of the sinusitis itself, eosinophil infiltration.