The high and low contrast LogMAR acuity of 20 patients with relatively flat (45.00D) central corneal curvatures, was measured when fit alternatively with Polymacon lenses of 20um, 35 um, and 55um center thickness; powers of -1.00D and -5.00D; and base curves of 8.4mm, 8.7mm, and 9.0mm. Analysis of Variance for repeated measures showed a trend for reduced visual acuity under all testing conditions with thinner center thickness lenses, and for steeper cornea/base curve fitting relationships. Interaction analysis indicated that flatter corneas exaggerated the lens induced effects, and that thinner center thickness lenses provided optimum visual acuity only when the correct base curve was used, even though the actual on-eye fit appeared acceptable. Thicker lenses were more resistant to visual degradation when an inappropriate base curve was used. These results suggest that visual acuity testing may be a more sensitive indicator of thin, low water, soft lens fitting than more traditional measures such as movement and centration.