Pisa Syndrome is clinically defined as the sustained lateral bending of the trunk worsened by a prolonged sitting position or by walking. Pisa syndrome, also termed lateral trunk flexion (LTF), has been rarely reported in patients affected by Parkinson's disease (PD) and, therefore, the pathophysiology has been poorly investigated. In some cases, the hyperactivity of paravertebral muscles contralateral to the leaning side has been interpreted as a sign of dystonia; however, it is well known that paravertebral muscles flex the trunk ipsilaterally. We systematically explored the pattern of muscular activation underlying the lateral flexion of trunk in 10 PD patients (mean disease duration: 9.2 ± 3.0 years) presenting LTF for 3.6 ± 2.1 years. EMG performed during stance and during left and right lateral trunk flexion showed a continuous ipsilateral muscular hyperactivity in three patients, while in the remaining ones there was no ipsilateral activity during standing and a tonic contraction of paravertebral muscles contralateral to the leaning side. In conclusion, this EMG study investigating the synergies of paravertebral muscles during dynamic conditions detected two different patterns with a typical dystonic activation in only a minority of cases. Possible pathophysiologic mechanisms and treatment approaches are discussed.