This study was done to determine the prevalence of visual impairment due to refractive errors and ocular diseases in lower middle class school children of Hyderabad, India. A total of 4,029 children, which included 2,348 males and 1,681 females, in the age range of 3 to 18 years from 9 schools were screened with a detailed ocular examination protocol. Among 3,669 children in whom visual acuity could be recorded, on presentation 115 (3.1%) had visual acuity < 6/18 in the better eye (equivalent to visual impairment), while 41 (1.1%) had visual acuity < or = 6/60 [corrected] in the better eye (equivalent to legal blindness) out of which 18 (0.5%) had visual acuity < 6/60 in the better eye (equivalent to economic blindness). Of 115 children who presented with initial visual acuity < 6/18, vision improved to > or = 6/18 with refraction in 109 (94.8%). No child was legally or economically blind after refractive correction. Prevalence of hyperopia was 22.6%, myopia 8.6% and astigmatism 10.3%. The prevalence of myopia was significantly higher among children > or = 10 years of age (P < 0.001). The maximum, mean and median values for myopia were 10.00, 1.35 and 0.75 D in the better eye. For hyperopia these values were 8.50, 0.65 and 0.50 D. The major causes for best corrected visual acuity < 6/9 in the worse eye for 51 (1.4%) children included amblyopia in 40 (1.1%), corneal diseases in 5 (0.1%), cataract in 2 (0.05%) and others in 4 (0.1%). Out of the total, 30 (0.7%) children had strabismus. These data support the assumption that vision screening of school children in developing countries could be useful in detecting correctable causes of decreased vision, especially refractive errors, and in minimising long term permanent visual disability.