Human postural control of upright stance sporadically can show an oscillatory behavior. Based on previous work, we assessed whether an abnormal tendency for such oscillations might contribute to the motor impairments in patients with basal ganglia dysfunction such as Parkinson's disease (PD). We investigated postural control during unperturbed stance in normal control subjects and in PD patients off and under treatment, focusing on stabilogram diffusion analysis (SDA) of the foot center of pressure (COP) excursions and conventional measures of the sway amplitude and velocity. We found abnormal 1 Hz body sway oscillation in the SDA curves of PD patients that differed significantly from the body sway typically observed in control subjects during quiet stance. The 1 Hz body sway oscillation was associated with abnormally large and fast sway in the patients off treatment. Under treatment with levodopa, with 'deep brain stimulation' (subthalamic nucleus) and even more so with combined treatment, the oscillations in the SDA curves vanished and the sway became slower. The loss of oscillation and reduction of sway velocity were highly correlated with the improvements of patients' clinical motor assessment score. However, sway amplitude was not correlated with the patients' motor assessment score and patients reported clinical improvement under therapy even though sway amplitude increased on average. A simple feedback model of the postural control system with abnormally large internal noise could predict experimental measures both on and off treatment. The off treatment condition was consistent with a high motor gain in the feedback loop, and the on treatment condition with a reduced motor gain.