Vestibular schwannoma (VS) tumour size has been considered the primary determinant of hearing outcomes and facial nerve function according to several published studies. Varying methods are used to estimate the size and volume of the VS with very little consensus amongst authors. The Koos classification was introduced in 1998 to stratify the tumours based on size, intrameatal extension and brainstem compression and is one of the more widely-used grading classifications. The current paper describes an assessment of the intra-and inter-rater reliability of the Koos classification system. The authors included a cross-section of magnetic resonance imaging studies of 40 patients with VS of varying sizes. They selected four raters to assign a Koos grade to a total of 50 scans. The study found that the level of inter-rater reliability was substantial while the intra-rater reliability was perfect for three and extremely high for one of the four raters. The authors conclude that the Koos classification system is significantly reliable and an effective means of characterising VS. This paper adds to the neurotological literature on VS although it does not suggest a definitive method to determine VS size.