Mucoceles of the paranasal sinuses are well-described complication of chronic sinusitis in adults. They are extremely rare in children and the main predisposing factor is cystic fibrosis (CF). We report a case of bilateral ethmoid and maxillary sinus mucopyoceles, associated with CF occurring in a six months old boy. The main symptom was a complete nasal obstruction. The marsupialization of mucocele was performed by endoscopic surgery. One year follow-up showed no recurrence.