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BACKGROUND Parkinsonian tremor usually starts asymmetrically. The mid-term prognosis of this lateralized tremor is unknown, as is the development of tremor in the contralateral arm. OBJECTIVE To investigate the occurrence of contralateral tremor in the Parkinson's Progression Marker Initiative database, with data available for 7 years. METHODS Tremor requiring treatment (TRT) was defined as any rest, postural or kinetic tremor with amplitude >1 cm (MDS-UPDRS score ≥2) as this criterion is commonly accepted as an insufficiently treated tremor. Tremor was analyzed by side mainly in the off-medication state. RESULTS At baseline, 348 (87.7%) of the 397 patients with Parkinson's disease had tremor at least on one side of the body. 183 (46%) had only mild tremors but 165 (41.6%) had TRT. 159 patients (40.1%) had lateralized TRT and six (1.6%) had bilateral TRT. Among patients with asymmetrical TRT, 40 patients (25.2%) developed contralateral TRT at 3 years, 49 patients (30.8%) at 5 years, and 61 patients (38.4%) at 7 years. The side more affected by tremor was also more affected by other cardinal signs. In 159 patients with initially asymmetrical TRT, tremor severity did not increase on the more tremulous side over the 7-year period. However, there was an increase in tremor on the contralateral side. This was associated with a clear increase in bradykinesia and rigidity on both sides. CONCLUSION The study findings may prove beneficial in counseling patients with TRT, and may also provide an explanation as to why the worsening of tremor is not correlated with overall disease progression.