Visual masking assesses visual perception and attention; it occurs when a visual stimulus (mask) interferes with the perception of a stimulus that the participant is trying to identify (target). A backward masking study (target presented before mask) was performed on 662 children without disabilities (338 females), aged between 6 and 17 years, in order to evaluate if performance varies with age. In the masking procedure 10 letters were presented through a tachistoscope as target stimuli. Fragments of letters oriented at random ('noise') represented the mask. A slight improvement of visual performance from the beginning of school age to 9-12 years of age was found. This paper gives normative data for the most important parameters which can be used as a standardized reference for the procedure employed. We also studied 113 children with epilepsy (56 females), aged between 5 and 19 years, who attended a mainstream school and had been seizure free for at least 2 years. Children were tested just before starting antiepileptic drug withdrawal and re-tested 1 year later; they were drug free for 3 months before the second test. These children showed, during and after treatment, only slightly worse results when compared with healthy children of the same age; after therapy withdrawal, their visual performance slightly improved but this was not statistically significant.