Asthenopia is a term used to describe different symptoms associated with the use of the eyes, such as pain, blurred vision, diplopia, headaches. Asthenopia is most often reported in association with near vision. Children with asthenopia complain of such symptoms particularly when reading and writing. Asthenopia is often divided into two main categories: refractive including refractive errors and anisometropia, and muscular, comprising strabismus and convergence insufficiency. Asthenopia due to accommodative problems has in the present studies been regarded as muscular asthenopia. In paper I the prevalence of asthenopia, refractive errors and binocular disorders was determined in a representative population of 216 Swedish schoolchildren aged 6 - 15 years. The prevalence of asthenopia was 23.1 %. The prevalence of hypermetropia and myopia changed with age, while astigmatism, convergence ability and strabismus did not. Accommodative insufficiency was more common in the older schoolchildren. Asthenopia was related to uncorrected visual acuity and refractive errors, and to accommodative insufficiency. Paper II described the orthoptic and ophthalmological findings in a group of 120 schoolchildren with asthenopia. The effect of asthenopia treatment was also evaluated. The most frequently occurring findings related to asthenopia were refractive errors, heterophoria and accommodative insufficiency. With appropriate treatment with glasses, prism or orthoptic exercises for 3 - 6 months, 112 out of the 120 children (93%) became asymptomatic. In paper III 49 schoolchildren with asthenopia due to accommodative insufficiency were assessed with the Visual Analogue Scale (VAS) and the grade of asthenopia was correlated with the degree of accommodative deficiency. The aim was to investigate if VAS grading of the asthenopic symptoms could be used as an instrument to indicate the level of improvement of accommodative insufficiency after treatment. A statistically significant reduction of asthenopic symptoms as graded with the VAS ...