We recorded resting state neuronal activity from the human subthalamic nucleus during functional stereotactic surgeries. By inserting up to five parallel microelectrodes for single- or multiunit recordings and applying statistical spike sorting methods, we were able to isolate a total of 351 single units in 65 patients with Parkinson's disease (PD) and 33 single units in 9 patients suffering from essential tremor (ET). Among these were 93 pairs of simultaneously recorded neurons in PD and 17 in ET, which were detected by the same (n=30) or neighboring microelectrodes (n=80). Essential tremor is a movement disorder without any known basal ganglia pathology and with normal dopaminergic brain function. By comparing the neuronal activity of the STN in PD and ET patients we intended to characterize changes of basal ganglia activity in the human disease state, that had previously been described in animal models of Parkinson's disease We found a significant increase in the mean firing rate of STN neurons in PD and a relatively larger fraction of neurons exhibiting burst-like activity compared to ET. The overall proportion of neurons exhibiting intrinsic oscillations or interneuronal synchronization as defined by significant spectral peaks in the auto- or crosscorrelations functions did not differ between PD and ET when considering the entire frequency range of 1-100 Hz. The distribution of significant oscillations across the theta, alpha, beta and gamma band, however, was uneven in ET and PD as indicated by a trend in Fisher's exact test (p=0.05). Oscillations and pairwise synchronizations within the 12-35Hz band were a unique feature of PD. Our results confirm the predictions of the rate model of Parkinson's disease. In addition, they emphasize abnormalities in the patterning and dynamics of neuronal discharges in the Parkinsonian STN, which support current concepts of abnormal motor loop oscillations in Parkinson's disease.