Orbital cellulitis is a potentially life-threatening condition. Compression of the optical nerve can cause total or partial loss of vision. Early diagnosis is crucial to prevent complications. In case of a unilateral sinusitis as cause of a unilateral orbital cellulitis complete clinical and dental examination combined with imaging are essential in diagnostics.A 53-year-old man presented with left eye movement impairment, intermittent diplopia and moderate swelling of the left lower eyelid. His diagnosis was post septal orbital cellulitis and despite administration of oral antibiotics no clinical improvement was observed. Orbital imaging by CT could not exclude a dental cause of his unilateral maxillary sinusitis. He was referred to the department of oral and maxillofacial surgery where clinical examination showed a dental cause. After removal of two decayed upper molars a complete recovery was accomplished.Odontogenic causes for unilateral orbital cellulitis should always be considered in diagnostics in adults. Clinical presentation and dental examination combined with adequate imaging can confirm the diagnosis.