To evaluate all nondiagnostic fine needle aspiration biopsy (FNAB) specimens from the thyroid gland with subsequent histopathologic diagnoses at Ottawa Civic Hospital. The criterion for specimen adequacy used in our institution was also reexamined to determine if it was too stringent. Review of 114 nondiagnostic FNAB samples from 91 patients with subsequent histopathologic diagnoses formed the basis of this study. Specimen adequacy was determined by the presence of 8-10 fragments of well-preserved follicular cells on at least two smears. Review of the 91 surgical specimens found 50 nodular goiters, 23 follicular adenomas, 6 macrofollicular adenomas, 5 cases of thyroiditis, 5 true cysts, 1 papillary carcinoma and 1 minimally invasive follicular carcinoma. Forty-two percent of lesions showed cystic change. In addition, nine cases of papillary microcarcinoma were diagnosed. A considerable difference in the rate (22% vs. 45%) of inadequate thyroid FNAB samples was identified among different groups of physicians at our institution. Ninety-eight percent of the patients with nondiagnostic FNAB of the thyroid gland had benign lesions. This finding encouraged us to continue using our criteria for adequacy because of the importance of a negative report. The higher rate of nondiagnostic thyroid aspiration in our series may reflect the varied experience of the different aspirators at our institution and/or the cystic nature of many of the lesions.