A rating scale for tardive dyskinesia was developed, consisting of nearly all signs seen by two groups of investigators over a 5-year period. Thirty-four items were included in the scale with a possibility of writing in idiosyncratic signs. The scale was shown to have good reliability and validity in studies carried out by both the New York and Boston groups. It is recommended as a suitable instrument for describing the breadth of tardive dyskinesia syndrome and also for quantifying the disorder. A second scale, "the abbreviated dyskinesia scale", contains 13 items which are more global than the items in the original scale. It also has been shown to be both reliable and valid. Its use is suited to situations requiring less extensively detailed assessments.