The reliability of subjects to perform to perimetry correctly should be carefully evaluated to interpret visual field examinations adequately. Clinicians generally agree that numerous false-positive responses to catch trials cause measured thresholds to be falsely high and numerous false-negative responses cause measured thresholds to be falsely low. We studied the effect of false-positive and false-negative responses on the outcome of visual field measurements. Of 47 eyes, the results of 106 stable glaucomatous visual field tests (Program G1, Octopus 201, Interzeag, Schlieren, Switzerland) with false-positive responses and no more than one false-negative response to catch trials were compared to the results of reliable visual field tests (no false-positive and no more than one false-negative response) performed on the same eye. Similarly, 60 stable visual fields with false-negative responses and no more than one false-positive response were used to study the effect of false-negative responses on visual field sensitivities. Linear regression analysis disclosed a mean sensitivity increase of 1.5 dB for every 10% of false-positive responses (r = .34, P = .000) and a mean sensitivity decrease of 1.2 dB for every 10% of false-negative responses (r = .26, P = .04). These results may be used to help reduce the magnitude of unexplained long-term fluctuation in visual field interpretation.