A prospective study of 407 patients with "cold" solitary thyroid nodule seen over 4 1/2 years and systematically operated upon was undertaken with the view of determining the pre-operative criteria of malignancy. Thirty-nine nodules were malignant. Since various methods of multidimensional statistical analysis failed to give satisfactory results, an attempt was made to base rational surgical indications on purely descriptive criteria. Neither the patient's age, morphology, personal or family history of thyroid disorders, nor the size of the nodule, the circumstances in which it was discovered and the presence of calcification made it possible to distinguish between malignant and non-malignant nodules. Cervical lymph node involvement, paralysis of the recurrent nerve, previous irradiation of the neck and site of the nodule equally had no discriminative value. Only two clinical findings, softness of the nodule and liquid content, were clearly associated with the absence of malignancy.