National Cancer Institute State of the Science Conference on thyroid fine-needle aspiration (FNA) summarized diagnostic terminology. Six diagnostic categories were proposed including "follicular lesion of undetermined significance" (FLUS). FLUS was defined as findings neither convincingly benign nor sufficiently atypical for a diagnosis of "follicular neoplasm" or "suspicious for malignancy." It was proposed that this category represent less than 7% of thyroid FNAs. A search of the cytology records at three University Hospitals was performed for the term FLUS or older equivalent terms. Usage of FLUS was compared between institutions and among pathologists. Surgical pathology outcome for FLUS cases was determined. Twenty-eight pathologists evaluated 6,872 cases at the three institutions. Use of FLUS varied among pathologists (2.5 to 28.6%). Frequency of use of FLUS among institutions varied from 3.3 to 14.9%. FLUS cases [127 of 673 (18.9%)] underwent surgical exploration with malignancy identified in 36 cases (28.3%) undergoing resection. Use of FLUS varied substantially among pathologists and institutions. FLUS category requires more rigorously defined morphologic criteria for it to become a useful guide in clinical management.