We evaluated the effects of the levo (l) enantiomer of amphetamine sulfate on cognitive function in multiple sclerosis (MS) patients. Using a counterbalanced within-subjects design, 19 MS patients received four single-dose administrations of placebo, 15 mg, 30 mg, or 45 mg of l-amphetamine. Neuropsychological tests measuring processing speed and memory served as the primary outcomes. Performance on tests of processing speed were improved following the 45 mg condition and the largest effects were observed on the Symbol Digit Modalities Test, which measures visual processing speed and working memory. While episodic memory test effects were in the expected direction, the findings were not statistically significant. These preliminary findings show promise for the use of l-amphetamine for the symptomatic treatment of slowed mental processing in MS. Further placebo-controlled trials are needed to confirm these findings.