To retrospectively compare the diagnostic accuracy and cytomorphologic features of thyroid lesions on ThinPrep (TP) (Cytyc Corporation, Boxborough, Massachusetts, U.S.A.) monolayer preparations with those of the conventional smear (CS) method on fine needle aspiration biopsy (FNAB). Slides of 145 TP and 145 CS consecutive cases of thyroid FNAB were retrospectively reviewed for the following features: amount and architecture of follicular cells, nuclear and cytoplasmic details, amount and quality of colloid, background blood, cyst fluid and macrophages, and lymphocytes and plasma cells. These were semiquantitatively scored 0-4 for each parameter. The TP slides more often had higher cellularity with flat clusters, while CS slides more often had 3-dimentional clusters. The CS slides displayed better morphology and more preserved follicular cells with intact cytoplasm and crisper nuclei, while TP slides revealed shrunken cells with fragmented cytoplasm and dark, often-naked nuclei. The amount of colloid was generally more abundant on the CS slides, while it appeared as small, dense droplets (thick colloid) or as folded tissue paper-like material (thin, watery colloid) on the TP slides. The CS more often had a bloody background obscuring the cells, while the TP slides had a clear background. Simple thyroid cysts were more often detected on TP than CS slides by the presence of cyst fluid and macrophages. The 2 methods had almost similar diagnostic rates for chronic thyroiditis (11% TP vs. 12% CS) and atypical/neoplastic lesions (3.4% each). The 2 methods had similar diagnostic correlation for colloid nodules (49% TP vs. 45.5% CS), but the nondiagnostic rate was lower in TP (24%) than CS slides (31%). Although there are cytomorphologic differences between the TP and CS methods, including better cellular preservation and details on CS, the TP method shows a lower nondiagnostic rate, similar diagnostic rate for chronic thyroiditis and atypical/neoplastic lesions, and slightly better diagnostic rate for colloid nodules. The 2 methods complement each other, and we strongly recommend that they both be performed on all thyroid FNAB cases.