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To perform a statistical evaluation with use of Bayes' theorem, evaluating the prognostic value of Doppler flow analysis, to determine the probability of cancer in thyroid nodules with a prior follicular biopsy specimen.The medical literature was reviewed to identify studies in which ultrasonographic Doppler flow analysis was performed on thyroid nodules with a prior fine-needle aspiration biopsy sample showing follicular cytologic features. For inclusion in the study, all nodules must have had pathologic confirmation of the final diagnosis. The pretest probability of cancer was considered to be the incidence of carcinoma within the population studied. The posttest or conditional probability was calculated by using Bayes' theorem.Two well-designed prospective studies met the criteria for inclusion. The sensitivity of demonstration of intranodular flow by Doppler analysis for detection of thyroid carcinoma ranged from 80% to 86%. The specificity of intranodular flow indicating cancer ranged from 85% to 89%. The incidence of thyroid cancer (pretest probability) ranged from 12% to 14%. Application of Bayes' theorem indicated that the posttest probability of thyroid cancer declined to 3% when no central intranodular flow was present and increased to nearly 50% in the presence of intranodular flow.Doppler flow analysis, when performed on thyroid nodules with follicular cytologic findings on fine-needle aspiration biopsy, can provide extremely useful information regarding the probability of malignant involvement. This information can be used in the decision-making process about the need for, and the extent of, surgical resection of follicular lesions.