PURPOSE: Children with amblyopia demonstrate modest improvements in both positional and visual acuities ( approximately 30%) after a short period (15 hours, 4000 trials) of perceptual learning. The present study was conducted to determine whether extended training is necessary for optimal treatment of amblyopia. METHODS: Two children, aged 9 and 12 years, with previously untreated severe amblyopia (Snellen acuity, 20/100-20/125) practiced a position-discrimination task repeatedly over 3 months (100 hours, >25,000 trials by each observer). The task was to judge which of three pairings of two groups of eight Gabor patches was misaligned. Positional noise was used to investigate the neural mechanisms involved in learning position discrimination. RESULTS: After practice both observers showed substantial recovery in both positional and letter acuities ( approximately 60% and 2-4 chart lines) and both also regained significant stereoacuity. In the first 20 hours, the recovery rate was comparable to that of 12 previously treated amblyopes. However, extending the treatment dosage for an additional 30 hours resulted in substantially greater plateau improvements. These improvements were primarily the results of a marked increase in sampling and processing ability and, to a lesser degree, to a decrease in spatial distortion. CONCLUSIONS: The results show that in two juvenile amblyopes, perceptual learning extended over an accumulated dosage of approximately 50 hours may be an efficient and effective adjunct to occlusion for reversing amblyopia. When combined with occlusion, perceptual learning may significantly speed up the time to recovery in children with amblyopia.