I used the laser interferometer to test 137 eyes to determine whether preoperative interference visual acuity tests could accurately predict postoperative Snellen visual acuities. In 60 patients with immature cataracts and normal maculas the postoperative Snellen visual acuity was predictable within one line in 53 patients and within two lines in seven others. In 29 other patients with cataracts and senile macular scars or macular degeneration without subretinal fluid postoperative Snellen visual acuity was predictable within one line in 26 patients and within two lines in two others. Several easily recognizable clinical conditions make the interferometer test unreliable. Those that produce false-positive responses are serous detachment of the sensory epithelium of the macula, cystoid macular edema, visual fields cut through fixation, amblyopia, macular holes or cysts, geographic atrophy of the pigment epithelium of the macula, and early postoperative retinal detachment. Those that produce false-negative responses are mature cataracts and testing without pupillary dilatation.