Protocols with demonstrated reliability have been established for the diagnosis of numerous movement disorders. whereas in the essential tremor (ET) literature, there is no discussion about the reliability of diagnostic protocols. Lack of knowledge of the reliability of diagnostic protocols in ET limits the use of these protocols because reliability is an essential requirement for scientific quality in data management. The objective of this study was to determine the reliability of a protocol for diagnosing ET. The protocol consists of a Tremor Interview, a videotaped Tremor Examination, and a diagnostic algorithm. Eighty-three subjects with ET, identified in a community-based health study in Washington Heights-Inwood, New York, were matched with 83 control subjects from the same community. These subjects and their relatives are being recruited to participate in the Washington Heights-Inwood Genetic Study of ET. Two hundred twenty-six subjects have been evaluated to date (35 ET cases, 40 controls, 151 relatives). All 226 underwent an 84-item Tremor Interview and 26-item videotaped Tremor Examination. Diagnoses (normal, possible ET, probable ET, definite ET) were independently assigned by two blinded neurologists specializing in movement disorders. The kappa statistic, k, was used to determine diagnostic agreement between these two neurologists. The concordance rate between two raters using diagnostic categories definite ET, probable ET. possible ET, and normal was 80%; kw = 0.84 (near perfect to perfect agreement). The concordance rate between two raters using two diagnostic categories (definite ET and normal) was 100%; k = 1.00 (perfect agreement). There was high correlation between the two raters' total tremor scores (r = 0.89, p < 0.00001). This diagnostic protocol is highly reliable. Research in ET would greatly benefit from diagnostic protocols with demonstrated reliability.