PURPOSE/HYPOTHESIS: Limited upper extremity (UE) motor recovery following stroke (ST) remains a problem for functional use of the limb in daily tasks. A few intervention studies have examined the effectiveness of a bimanual (BI) approach to paretic (P) UE recovery although results have been equivocal. With one exception, motor control studies report behavioral adaptation of the nonparetic (NP), but not the P UE in BI compared to unimanual (UNI) movements. Facilitation of the P UE has been shown for a single-joint bilateral task but has not been demonstrated for more functional multi-joint movements. The purpose of this experiment was to determine the necessary task conditions for P UE facilitation in BI compared to UNI multi-joint goal-directed aiming movements. NUMBER OF SUBJECTS: Twenty-nine patients status-post unilateral ST (18 male; mean age = 63.0 +/− 12.0 years; 17 with left hemisphere lesion; mean time since onset = 29 +/− 22 months) and 28 healthy control subjects (HC) (17 female; mean age = 68.0 +/− 16.9 years) participated. Mean UE Fugl-Meyer motor score for the ST group was 60.0 (49-66). MATERIALS/METHODS: Participants performed 20-trial blocks of a rapid goal-directed aiming movement in three different BI conditions: 1. Simple bimanual to a single target (BISAME), 2. Complex bimanual to two different distance targets: P-near distance, NP-far distance (BIPNEAR) and 3. Complex bimanual to two different distance targets: P-far distance, NP-near distance (BIPFAR) in response to an LED signal. Additionally, for each separate BI condition, participants also performed 20-trial blocks of UNI aiming with their P and NP UE. HC participants performed UNI aiming with the left UE analogous to the P UE of the ST participants. Peak resultant velocity (PRV) was derived from displacement data recorded from electromagnetic sensors attached to each hand. Mean PRV was calculated per condition and limb for each participant. Separate Group x Limb x Aiming Type repeated measures ANOVA determined differences between group (HC, ST), limb (P, NP) and aiming type (UNI, BI) for the three aiming conditions. RESULTS: P UE PRV was greater during BI compared to UNI aiming for two of the three conditions: BISAME (UNI: 156 +/− 33 cm/sec, BI: 163 +/− 35 cm/sec) and BIPFAR (UNI: 158 +/− 31 cm/sec, BI: 164 +/− 31 cm/sec) but not BIPNEAR (UNI: 157 +/− 32 cm/sec, BI: 152 +/− 30 cm/sec), (p < .05). This increase in P UE PRV for BI compared to UNI aiming stands in contrast to the decrease in PRV observed for the NP UE in BI compared to UNI aiming and to no difference between UNI and BI aiming for either UE of the HC group. CONCLUSIONS: This is the first report of a P UE facilitation effect in a multi-joint BI task. This effect was not observed for every BI condition, suggesting that the task demands of the P UE also contribute to this facilitation effect. CLINICAL RELEVANCE: These data suggest a BI intervention approach may enhance P UE movements and also demonstrate the importance of delineating critical features that underlie that facilitation in designing task practice protocols for specific intervention studies.