1. We measured the contrast sensitivity of 18 children, 4-13 years of age, who were born with a dense and central cataract in one (n = 10) or both (n = 8) eyes. The cataracts had been surgically removed and the aphakic eyes fitted with contact lenses (or, in two cases corrected with glasses). The period of deprivation ranged from 3 to 29 months. For comparison, we tested 23 comparably-aged children: 20 with a normal visual history, and three who had had a normal early history but then had incurred a traumatic cataract after the age of 5 years. 2. Compared with the normal and traumatic controls, eyes treated for congenital cataract showed a loss of sensitivity which increased with spatial frequency. This loss was generally much more profound than that typically reported for strabismic or anisometropic amblyopes but was much less severe than that which has been reported for eyes deprived by congenital cataract throughout childhood. 3. Among the children treated for congenital cataract, the highest sensitivity was achieved by eyes with the shortest deprivation, providing, in unilateral cases, the normal eye had been patched regularly following cataract removal and contact lens fitting. 4. Monocular deprivation from birth resulted in a greater loss of sensitivity than did binocular deprivation, and only after monocular deprivation was the degree of loss dependent on temporal frequency. For the binocular cases, the loss was always a constant proportion of normal sensitivity, regardless of temporal frequency. 5. Like results reported for comparable deprived animals, our findings suggest different pathophysiological bases for amblyopia from bilateral versus unilateral form-deprivation. With bilateral congenital cataract, form deprivation itself may be the principal amblyogenic agent. With unilateral congenital cataract, form deprivation and competition from the non-deprived eye both may effect the amblyopia.