The study was aimed to investigate the impact of using standard criteria for assessing specimen adequacy and diagnostic terminology (CAST) on fine-needle aspiration (FNA) diagnosis and clinical management of thyroid nodules. The study included similar numbers of FNAs performed in 2 year before (group A) and 1.5 year after (group B) implementing the standard CAST. In comparison to group A, group B showed a significantly lower rate of nondiagnostic specimens (RND) (16.1% vs. 21.6%, P <or= 0.01) and rate of descriptive diagnoses (RDD) (3.8% vs. 14.5%, P <or= 0.001) and greater non-neoplastic (70.0% vs. 64.1%, P<0.05) and follicular cell lesions (7.4% vs. 4.3%, P<0.05) but a similar percentage of neoplastic diagnoses. The rate of surgical follow-up (RSF) was significantly higher in group B than in group A, overall (21.6% vs. 17.0%, P<0.05), or in subgroups of non-neoplastic (12.6% vs. 5.4%, P<0.01) and neoplastic categories (81.0% vs. 61.0%, P<0.05). The rate of cytohistologic concordance was higher in group B although the difference was not statistically significant. We concluded that use of the standard CAST on FNA diagnosis of thyroid nodules significantly reduced RND and RDD, providing more consistent diagnoses among the pathologists as well as better and more uniform communication between the pathologists and the clinicians. Furthermore, the cytohistological concordance was slightly better after CAST implementation, indicating that the improvement of diagnostic consistency among pathologists did not sacrifice the diagnostic accuracy.