The Koos classification is a grading system used often for preoperative evaluation of acoustic tumours on imaging studies. It indirectly correlates to the size of the tumour. Size of the vestibular schwannoma is often considered the main determinant for hearing preservation and facial nerve function, but outcomes of facial nerve function depend on other factors too. For example, tumour surface in contact with the tumour, stickiness of the tumour to the facial nerve, etc. No reliable methods have been described to predict facial nerve function after treatment. The authors of this study assessed the validity of the Koos grading system with respect to facial nerve function after treatment of vestibular schwannomas in 81 patients. Patients treated with surgery or stereotactic radiation were included. The study showed a weak correlation between House Brackman score and Koos classification at the first postoperative visit. It did find, however, that there were higher odds of facial nerve dysfunction with higher Koos grades (III/IV) compared to lower grades (I/II). The authors concluded that the Koos classification did not predict the presence or absence of facial nerve dysfunction in their study population.