Background: Activities of daily living demand simultaneous performance of two or more tasks. Dual task effect (DTE) is the relative change in performance of tasks executed individually versus simultaneously and can be quantified: DTE%= (dual task performancesingle task performance)/single task performance. Attention is a limited resource and is it logical to posit that DTEs of cognitive-motor tasks are negative, especially in people with multiple sclerosis (PWMS) who may experience both cognitive and motor impairments. Objectives: Primary aim: Examine dual cognitive-motor task performances in PWMS. Hypothesis: PWMS would experience larger negative DTEs in cognitive and motor performances than controls. Methods: Thirteen PWMS (mean 48.2 years, mean Patient Determined Disease Steps Scale 1.92) and 13 matched controls (mean 46.5 years) performed a non-fatiguing, submaximal, isometric force steadiness task of the ankle dorsiflexors, muscles that contribute to standing balance, and the Paced Auditory Serial Addition Test (PASAT) under two conditions, the PASAT-3 and PASAT-4 in which stimuli were presented at 3 or 4 second intervals, respectively. Steadiness was expressed as coefficient of variation (CV%). Dual task effects on cognitive and motor performances were calculated. Comparisons were made with repeated measures ANOVA and t-tests. Results: Despite similar strength (Control=15.08 Nm, MS=13.54, p=.503), PWMS were less steady in both single task (CV% Control=1.48, MS=2.36, p<.01) and dual task conditions (PASAT-3 CV% Control=1.94, MS=2.83, p<.01; PASAT-4 Control=1.71, MS=2.49, p=.013). Both groups had negative motor DTEs that did not differ between groups (DTE% PASAT-3 Control=-29.47, MS=-42.66, p=.340; PASAT-4 Control=-20.37, MS=-24.58, p=.156). For cognitive measures, controls outperformed PWMS on single task PASATs (Accuracy% PASAT-3 Control=93.47, MS=80.13, p<.01; PASAT-4, Control=95.77, MS=84.13, p<.01). Compared to controls, PWMS had similar cognitive DTEs for PASAT-3 (DTE% Control=-9.39, MS=-13.27, p=.970), but larger negative DTEs on PASAT-4 (DTE% Control=-4.39, MS=-11.47, p=.012), suggesting that PWMS did not benefit from additional processing time in the PASAT-4. Conclusions: Controls and PWMS experienced negative DTEs on cognitive and motor performances. Although PWMS had lower baseline scores for single tasks, relative decline was only greater in PWMS when cognitive tasks were performed at a slower pace. Thus, performance declines may be dependent on available processing time.