Objective: The present experiment was designed to better understand cognitive fatigue (CF) in Multiple Sclerosis (MS) by testing a number of competing hypotheses against each other. The hypotheses were: 1) CF is a result of Cognitive Load, 2) CF is a result of Cognitive Domain, or 3) CF is a function of Temporal Onset. An Interactive CF Hypotheses was also possible, whereby CF may result from an interaction between the different factors. Method: 56 patients with MS and 23 Healthy Controls participated. Experimental manipulation included Cognitive Domain (Processing Speed vs Working Memory), Cognitive Load (High vs Low), and Block (Time/Run 1, 2, 3, 4). Domain and Load were counterbalanced and manipulated within participants. Subjective CF was measured using the Visual Analogue Scale at baseline and after each block. Results: Data were analyzed using a 2 (Group: MS vs Healthy Controls) x 2 (Cognitive Domain) x 2 (Cognitive Load) x 4 (Run) Mixed ANOVA. Subjective CF was higher for the Processing Speed task (p < .001), increased across Runs (p < .001), and was higher in the MS group (p = .01). The Run by Group interaction (p = .02) demonstrates higher subjective CF for the MS group across time. Subjective CF was not correlated with behavior (all p's = n.s). Conclusion(s): CF was higher for both groups in the processing speed task. Irrespective of Cognitive Load, CF increased as a function of time on task and this was magnified for the MS group. CF was independent of behavior. These data support the hypothesis that CF occurs in MS as a function of Temporal Onset.