Unilateral STN-DBS significantly improves the performance of contralateral sequential arm movements. Whether unilateral STN-DBS also improves ipsilateral sequential movement is unclear. In this study in unmedicated parkinsonian patients, we tested the effect of unilateral STN-DBS on the performance of ipsilateral sequential movements and compared it with the performance of contralateral sequential movements. Three-dimensional movements were recorded with the ELITE system and three kinematic variables were considered: total movement time (TMT), total inter-onset latency (IOL), and spatial accuracy. Unilateral STN-DBS significantly decreased TMT in the contralateral arm and only tended to do so also in the ipsilateral arm, whereas it significantly decreased IOL and worsened spatial accuracy only on the contralateral side. Before unilateral STN-DBS a positive correlation was present between the clinical impairment and the TMTs in the contralateral and ipsilateral sides. After unilateral STN-DBS the UPDRS scores improved in the contralateral and to a lesser extent also in the ipsilateral side. Correlation analysis between clinical and kinematic data showed no differences between the contralateral and ipsilateral sides. Our kinematic findings show that after STN-DBS parkinsonian patients' performance of a sequential motor task improves significantly on the contralateral but only tended to do so on the ipsilateral side. Ipsilateral changes can be explained by the observation that the output structures of the basal ganglia send large ipsilateral and less dense contralateral projections to the thalamus.