Any intervention to prevent serious amblyopia is based on the knowledge about normal versus subnormal visual development. Our ability to predict with high degree of certainty which children will develop amblyopia will be dependent on the characteristics of various risk factors for initiating the development of squint or amblyopia. We have used longitudinal studies of population based cohorts of young children to define some of these risk factors such as refractive errors. Three hundred and ten children with an astigmatism greater than or equal to 1.0 D at one year of age were refracted yearly between the age one and four years. Astigmatism and anisometropia were found to be highly variable during infancy and early childhood. Longitudinal follow-up seems to be needed to separate the normal from the abnormal refraction development, which initiates the development of the amblyopia. Children with constant or increasing astigmatism or anisometropia between one and four years were 'at risk'. In parallel we have studied important factors for successful treatment of amblyopia. Based on these findings we conclude that a population screening at four years of age seems to be advantageous in Sweden in order detect and successfully treat most cases of amblyopia.