This study assessed the prevalence and causes of unilateral visual impairment in the urban population of Hyderabad city as part of the Andhra Pradesh Eye Disease Study. Stratified, random, cluster, systematic sampling was used to select 2,954 subjects from 24 clusters representative of the population of Hyderabad. Eligible subjects underwent detailed eye examination including logMAR visual acuity, refraction, slitlamp biomicroscopy, applanation tonometry, gonioscopy, dilatation, cataract grading, and stereoscopic evaluation of fundus. Automated threshold visual fields and slitlamp and fundus photography were done when indicated by standardised criteria. Unilateral visual impairment was defined as presenting distance visual acuity < 6/18 in the worse eye and > or = 6/12 in the better eye, which was further divided into unilateral blindness (visual acuity < 6/60 in the worse eye) and unilateral moderate visual impairment (visual acuity < 6/18-6/60 in the worse eye). A total of 2,522 subjects (85.4% of eligible), including 1,399 > or = 30 years old, participated in the study. In addition to the 1% blindness and 7.2% moderate visual impairment (based on bilateral visual impairment criteria) reported earlier in this sample, 139 subjects had unilateral visual impairment, an age-gender-adjusted prevalence of 3.8% (95% confidence interval 2.7-4.9%). The major causes of this visual impairment 39.9% were refractive error (42.9%), cataract (14.4%), corneal disease (11.5%), and retinal disease (11.2%). Of this unilateral visual impairment was blindness. The major causes of unilateral blindness were corneal disease (23.2%), cataract (22.5%), retinal disease (18%), and optic atrophy (12.9%). On the other hand, the predominant cause of unilateral moderate visual impairment was refractive error (67%) followed by cataract (9%). Of the total unilateral visual impairment, 34.3% was present in those < 30 years old and 36.2% in those 30-49 years old. Unilateral visual impairment afflicts approximately 1 in 25 persons in this urban population. A large proportion of this unilateral visual impairment is present in younger age groups. The causes of unilateral visual impairment, like those of bilateral visual impairment in this population, are varied, suggesting therefore, that in addition to the current focus of eye care in India predominantly on cataract, other causes of visual impairment need to be addressed as well.