There is substantial variation in the reported rates of different diagnostic categories in The Bethesda System for Reporting Thyroid Cytopathology (TBS). Specifically, the relationship between the nondiagnostic (ND) and atypia of undetermined significance (AUS) categories has not been closely examined previously. Data from published series in the literature and from 2 separate hospitals with more than 15,000 thyroid aspirates were reviewed. The AUS and ND rates were consistently negatively correlated when analyzed by year, aspirator, and cytologist. The strongest correlation was with cytologists (P < .0003). Absolute ND rates decreased by 1% for every 3.5% increase in AUS, implying the existence of a discrete population of cases that cytologists will classify as ND or AUS. As such, AUS and ND are not independent variables. Awareness of this relationship may be useful for laboratories and individual cytopathologists for refining the use of TBS.