The results of clinical trials with a simple system for automating visual acuity measurements are presented. An inexpensive microcomputer: (1) presents acuity targets on a television screen, (2) uses patients' responses entered through a remote response box to control a staircase testing procedure, and (3) prepares a printed record of a patient's mean visual acuity and its standard deviation. The automated method: (1) offers a stable measure of visual performance, (2) provides an immediate analysis of the statistical significance of changes measured in visual acuity from one office visit to the next, and (3) gives results that are in general agreement with conventional chart testing methods when high contrast targets are used on the television screen. In addition, when medium and low contrast targets are used, the automated method provides a variable contrast acuity profile (VCAP), which describes visual performance at contrast levels that are vital to a patient's performance in daily life but not examined in conventional chart testing. VCAPs are measured clinically, and it is shown that the extent that a patient's acuity declines with decreasing contrast cannot be predicted from the patient's high contrast acuity. Automated variable contrast acuity testing is found to provide a simple, reproducible, clinically practicable method of documenting and quantitating the extent of a patient's visual difficulties at medium and low contrasts.