Ethmoid sinuses are one of the first formed of all the paranasal sinuses, therefore they are most frequently affected by inflammation when it comes to children. The diagnosis of acute ethmoiditis is based mainly on clinical presentation. Due to the specific anatomical conditions of the ethmoid sinuses, inflammation is spreading fast onto the orbital structures and the surrounding tissues. It is estimated that in approximately 91% of cases, orbital cellulitis is related to sinusitis. This work aims at drawing attention to the importance of early diagnosis and appropriate treatment of a common illness that is acute rhinosinusitis. The presented case report discusses the course of the disease in a 6-year-old boy who additionally developed orbital cellulitis – a rare complication that concerns approximately 0.5–3.9% of all cases. We can distinguish two types of orbital complications: preseptal cellulitis (anterior to the orbital septum) and posterior to the orbital septum ex. orbital cellulitis (inside the septum). The first group contains inflammatory eyelid oedema, relatively mild and most frequently observed, characterized by favourable prognosis. The complications belonging to the second group are decidedly less frequent and more serious. These are: orbital cellulitis, subperiosteal abscess, orbital abscess and cavernous sinus thrombosis. What helps to differentiate between the two groups are proptosis, eye movement disorder and decreased visual acuity. The systemic treatment consists in the parenteral administration of broad spectrum antibiotics. Despite available modern treatment options one should not forget the fact that late intervention may lead to septic complications such as meningitis, cavernous sinus thrombosis or osteomyelitis. Moreover, discontinuation of treatment may cause permanent, but very rare sequelae including loss of vision and glaucoma. Therefore, acute ethmoiditis is to be treated as a paediatric and rhinological emergency condition.