According to several large studies, the surgical pathologist renders a non-neoplastic diagnosis in ∼20–40% of thyroid fine-needle aspiration (FNA) cases reported as follicular neoplasm. This study analyzes the cause of this poor correlation between cytology and histology. Cases consisting of oncocytic (Hurthle) cells were excluded from study. During the study period from January 1996 to April 2010, histologic follow-up was available for 234 of 670 cases (34.9%) reported as follicular neoplasm on ultrasound-guided thyroid FNA. Sonographic and Doppler data were available in all cases and included nodule location, size, echogenicity, and vascularity. Of the 234 aspirates with follow-up, surgical pathology reported 130 cases (55.6%) of follicular adenoma, 15 cases (6.4%) of follicular carcinoma, 14 cases (6.1%) of follicular variant of papillary carcinoma, and 75 cases (32.3%) of nodular goiter. Recuts of those index nodules reported as nodular goiter were examined independently by two pathologists using the 2× objective lens. Adenomatoid nodule was defined as an insufficiently encapsulated “blue” nodule of increased nuclear density when compared with the surrounding thyroid. Of the 75 cases reported as nodular goiter, 60 index nodules (80%) fulfilled the described criteria for adenomatoid nodule, while 15 did not. In conclusion, adenomatoid nodules are the main cause of poor histologic correlation with follicular neoplasm reported by FNA. If “increased nuclear density at scanning magnification” were adopted by surgical pathologists as the major diagnostic criterion for follicular adenoma rather than encapsulation, noncorrelated cases would be reduced from 32 to 6.4%. Diagn. Cytopathol. 2010. © 2010 Wiley-Liss, Inc.