The crowding effect, defined as the ratio of visual acuities for letters presented in Snellen (i.e., line) format and isolated-letter format, was measured in the amblyopic eyes of 15 children and 15 adults with unilateral amblyopia. Normal limits were based on data from 20 children and 20 adults with no history of amblyopia. The crowding effect was compared for letters of high (96%) contrast and for letters of low (11%) contrast. We report that amblyopia can differentially affect line and isolated-letter acuity for both high- and low-contrast letters. For the patients that show an abnormal crowding effect, this effect can be: (1) significantly stronger, (2) significantly weaker, or (3) not significantly different for high- than for low-contrast letters. These findings are consistent with the hypothesis that the crowding effect is contrast-dependent in some amblyopic eyes of both child and adult unilateral amblyopes.