Purpose: The purpose of this study is the evaluation of fixation disparity curve (FDC) parameters with an instrument that includes a central fusion lock (the modified near Mallett unit) and another without one (the Wesson fixation disparity card) to determine which is useful for diagnosis of symptomatic from asymptomatic subjects. Materials and Methods: In this analytical-descriptive study, 100 students were selected randomly and divided into symptomatic and asymptomatic groups. FDC parameters were determined with the Wesson card and the modified near Mallett unit for each subject and compared in symptomatic and asymptomatic groups. Data were analyzed by Wilcoxon, Mann-Whitney U, Spearman correlation coefficient and Chi-square tests. Results: The mean slope, y-intercept, and x-intercept with the Mallett unit and the Wesson card were significantly different in the symptomatic and asymptomatic groups (p < 0.001). Significant correlations were obtained between the Mallett unit and the Wesson card in y-intercept, x-intercept and slope (p < 0.001). There was a significant difference in the distributions of fixation disparity curve types in the two symptom groups with the Mallett unit (p = 0.01) and the Wesson card (p = 0.002) by Chi-square test. Conclusions: Among symptomatic participants in this study, both Type I and Type III FD are more common than previously thought, but depend upon the method used to measure it. The x-intercepts were on average displaced in the base-in direction, y-intercepts were shifted in the exo direction, and the slopes were steeper with the Wesson FD card compared with the modified near Mallett unit. This may be related to the design of two devices. These differences were more significant in the symptomatic group. © 2012 Informa Healthcare USA, Inc.