We investigated the applicability [123I]FP-CIT SPECT for the assessment of the rate of dopaminergic degeneration in PD. Twenty early-stage PD patients (age range 43-73 yr; mean age 55.4) were examined twice, a mean of 12 months apart. The mean annual change in the ratio of specific to nonspecific [123I]FP-CIT binding to the striatum was used as the outcome measure. The mean annual decrease in striatal [123I]FP-CIT binding ratios was found to be about 8% (of the baseline mean). In order to demonstrate a significant effect (p < 0.05) of putative neuroprotective agent with 0.80 power and 50% of predicted protection within 2 years, 36 patients are required in each group, when the effects are measured by means of changes in [123I]FP-CIT binding ratios in whole striatum. Our findings indicate that [123I]FP-CIT SPECT seems to be a useful tool to investigate the progression of dopaminergic degeneration in PD and may provide an objective method of measuring the effectiveness of neuroprotective therapies.