Previous fMRI studies using motor tasks yielded conflicting results concerning the activation pattern in Parkinson's disease (PD) patients. Possible explanations of these discrepancies include differences in the clinical features of the examined patients and in the executed tasks and incomplete task monitoring. We evaluated with fMRI 20 patients with untreated de-novo PD and 11 healthy controls with a simple motor task consisting of self-paced continuous right hand-tapping. The task was monitored on-line with a dedicated device which measures the strength and frequency of the tapping. Fifteen patients performed the task correctly. The frequency was not significantly different, whereas force was slightly different between patients (26.4+/-3.0 N) and controls (28.5+/-2.4 N) (p=0.046, Mann-Whitney U-test). After insertion of the subject's frequency and force as covariate variables in the model, PD patients compared to controls showed areas of significantly [Z statistic image>5.1 and p< or =0.05 (corrected) cluster significance] lower activation in the left primary sensorimotor (SM1) cortex and cerebellum and higher activation in the left temporal-parietal cortex adjacent to the SM1 and in right SM1. Furthermore in PD patients the disease severity evaluated with the Hoehn and Yahr staging system correlated significantly [Z statistic image>2.3 and p< or =0.05 (corrected) cluster significance] with activation of left SM1 and supplementary motor area and cingulum, bilaterally. The mixed pattern of decreased and increased cortical activation in de novo PD patients possibly reflects the coexistence of cortical deafferentation, and compensatory phenomena by cortico-cortical circuits.