The treatment strategy of an incidentally detected thyroid nodule is controversial. The aim of this study was to establish management criteria for thyroid incidentalomas by defining and formulating the risk factors predicting thyroid malignancy. A prospective database containing 815 consecutive patients who underwent a thyroidectomy for nodular thyroid disease, between January 1992 and May 2003, was studied. Multivariate analyses demonstrated that the independent clinical predictors of malignancy were a fixed nodule and cervical lymphadenopathy on palpation, a euthyroid patient, and a patient age <23 years or >45 years; and independent nodule features significantly associated with malignancy were punctuate calcifications, irregular nodule margin, solid appearance on ultrasonography, and solitary nodule in an euthyroid patient. Using the regression coefficients of four independent ultrasound (US) features, a malignancy risk score of a nodule was calculated as follows. Depending on the score of a thyroid nodule, a simple follow-up, a US-guided fine needle aspiration biopsy or a thyroidectomy, may be offered for management.