A multivariate analysis of 47 thyroid nodules and 34 diffuse goiters has been employed to determine the cause of failure in thyroid biopsy under ultrasonography. In the thyroid nodule cases, 30 percent were found to have been misdiagnosed or judged to be an inappropriate sampling. The most important factor leading to failure is a size of tumor. Only one fourth of the small tumors of less than 1 cm were correctly diagnosed and, in cases of a relatively large cystic tumor or a peripherally located nodule, an exact diagnosis was difficult, though 90 percent of the diffuse goiters were difficult, though 90 percent of the diffuse goiters were correctly diagnosed by this method. This result indicates that needle biopsy of the thyroid gland is reliable in cases of a diffuse goiter but problems remain to be resolved in cases of small or cystic tumors and peripherally located nodules.