BACKGROUND: Reliable epidemiological data on the prevalence and causes of visual loss in children are difficult to obtain, but are essential for planning. No such data are available from Mongolia. AIM: To determine the prevalence and causes of severe visual impairment and blindness (SVI/BL) in children from a defined area of Mongolia, using several methods of identification. METHODS: Children with presenting visual acuities of <6/60 in the better eye who lived in 10 of the 18 provinces (Aimaks) were identified 1) by family doctors 2) in the school for the blind 3) by visiting eye departments in the capital. All eligible children were examined (or data extracted from hospital records) and the cause of visual loss determined using the WHO classification system. RESULTS: Sixty-four children with SVI/BL before refraction were identified who lived in the 10 study Aimaks. They were recruited by family doctors (52); by home visits (3); from hospital records (4); or from the school for the blind (5). The prevalence of SVI/BL before refraction was 0.19/1,000 children (95% CI 0.16-0.22), decreasing to 0.16/1,000 after refraction (95% CI 0.13-0.19) but there was considerable variation from Aimak to Aimak. The major causes of SVI/BL were lesions of the lens (34%), central nervous system disorders (19%), lesions of the whole globe (e.g. microphthalmos) (14%), and retinal conditions (12.5%). Hereditary factors were responsible for 27% of causes, and 17% of children were blind following acquired conditions of childhood. The underlying cause could not be determined in 48%. The causes of SVI/BL was analysed in a further 16 children who lived outside the study Aimaks to compare the causes in children in special education with those not in schooling, and by age. CONCLUSION: The prevalence estimate obtained was lower than anticipated, and possible reasons are discussed. The pattern of causes of SVI/BL is similar to that in children in schools for the blind in China, but is very different from other Asian countries. Meningococcal meningitis was the most common preventable cause of SVI/BL, and immunisation is being considered. Other preventable causes were rare, and the majority of children needing surgical intervention had already been identified and referred for treatment. The control of blindness in children could possibly be improved by better management of conditions requiring surgery, and by the provision of low vision devices.