We studied interobserver variability (IV) in the assessment of thyroid fine-needle aspiration (FNA). We limited our cases to those showing predominantly colloid and follicular cell groups. Twenty cases of thyroid FNA diagnosed by 1 experienced cytopathologist were reviewed by 4 other cytopathologists who made their own diagnoses while unaware of the original diagnoses. Two cytopathologists then assessed the cytologic features of the 20 cases. IV was calculated for noncollapsed and collapsed diagnoses. Diagnoses and observer agreement were compared with cytologic features. There was little correlation among observers regarding the diagnosis of follicular "lesion" vs "neoplasm." IV was somewhat poor before data were collapsed to treatment recommendations (kappa = 0.35) but was relatively good when data were collapsed (kappa = 0.65). Cellularity, cyst change, and amount of colloid correlated with treatment recommendations; no specific features correlated with poor performance. Thyroid FNA shows good interobserver agreement in the diagnoses of lesions showing predominantly colloid or follicular cells (when collapsed). We speculate that IV is poor in some cases owing to difficulty assessing thin colloid, some lack of agreement regarding criteria for adequacy, and a possible "gray zone" that might exist with lesions showing colloid and abundant follicular cells.