Immediate adequacy assessment (IADA) during fine-needle aspiration (FNA) is not universal and the optimal number of passes has not been well determined. The aim of this study was to evaluate the nondiagnostic rates (NDR) with and without the IADA forthyroid aspirates. Subsequent cytological and surgical follow-up were reviewed for non-diagnostic cases. In addition, we evaluated the number of passes performed in each FNA to determine the optimal number. Retrospective analysis of NDR was performed on 883 thyroid FNA specimens retrieved through a Computer SNOMED Search from our files between January 2001 to December 2003. For FNAs with IADA, one Diff-Quick and one fixed smear for each pass were prepared, and the needle was rinsed in CytoLyt solution for a ThinPrep and/or a cell-block. FNAs without IADAwere received in CytoLyt solution, from which a ThinPrep and a cell-block were prepared for each case. Of the total 883 cases, 443 were performed with IADA, of which 417 cases were diagnostic. The remaining 440 cases were performed without IADA, of which 300 cases were diagnostic.NDR for IADA was 5.9% (26 cases-group-I)compared to 31.8% (140 cases-group-II)without IADA. In group-I, 5 cases were followed-up by repeat FNA, 10 cases by surgical resection, and 11 cases received no tissue follow-up. In group-II, 23 cases were followed-up by repeat FNA, 36 by surgical resection, and 82 cases received no tissue follow-up. Interestingly, follow-up in group-I did not reveal any missed malignancy, while that in group-II resulted in a malignant diagnosis in 13.8% (8 cases). We also found that the optimal number of passes with least NDR was 4-6 passes.NDR was 25% for < 3 passes, 11% for 4 passes, 5.2% for 5 passes, 1.4% for 6 passes, and 2.5% for 7 passesor more. IADA significantly reduces the NDR and increases the sample adequacy for diagnosis. Optimal number of passes is 4-6 passes, and additional passes did not improve the diagnostic rate. Our study also emphasizes the significance of repeat FNA or histological follow-up for nondiagnostic cases, especially for those without IADA.