We studied 207 glaucoma patients (207 eyes) who had two successive visual tests on the Humphrey Field Analyzer to determine the association of unreliability (> 10% incidence of a missed catch trial test) with threshold variation. We found that in patients where one visual field showed > 30% false negative or positive errors, and the other visual field also was unreliable, a significant increase in the variance of the mean defect existed from reliable patients (p < 0.05). Of patients who had only one of two visual fields which was unreliable (from false negative errors), only those with marked glaucomatous visual field loss (< -15 dB depth of defect) showed a greater variance in mean defect between examinations over reliable patients (p < 0.05). In total, 41 patients (19.8%) had unreliable visual fields associated with statistically increased threshold variation. This study suggests that patient reliability should be considered when interpreting changes in threshold between automated visual field examinations.