Is there a relationship between Parkinson's disease and essential tremor?

The issue of whether there is a relationship between essential tremor (ET) and Parkinson's disease (PD) is controversial partly due to the confusion regarding the accurate diagnosis of these conditions. The presence of postural tremor, which often occurs in PD, by itself is insufficient for the diagnosis of ET. Most epidemiological studies have shown that there is no association between these two conditions. Some studies, but not others have found a higher prevalence of tremor in family members of PD patients. Clinical, positron emission tomography scan, and neuropathological data have failed to show any relationship between these two conditions. It is concluded that there is no relationship between ET and PD and that when these two conditions are seen in the same patient, this represents a chance occurrence of two common diseases.