To identifiy categories of potentially neoplastic, nonpapillary follicular lesions of the thyroid that would reduce the number of lobectomies for nonneoplastic disease. The literature regarding fine needle aspiration (FNA) of follicular lesions is difficult to interpret largely due to the poor standardization of diagnostic categories. Based on our categorization scheme, which is directly comparable to the new system proposed by the National Cancer Institute (NCI), we quantitatively evaluated the cytomorphologic features of 99 "atypical" cases. Histologic evaluation revealed a 63% neoplasia rate and a 26% malignancy rate. Lesions with an abundance of macrophages and colloid on FNA were significantly less likely to be neoplastic or malignant. The strongest predictors of any neoplasia included absence of colloid and increased cellularity, which remained significant even after adjustment for all selected variables. Unlike similar studies, ours statistically significant variables that may assist how to manage follicular lesions better. Further, our current diagnostic scheme is equivalent to that recently proposed by the NCI, and, herein, we not only illustrate its application in clinical practice but also highlight potential pitfalls of this system.