Current pathological classifications of thyroid carcinoma only partially reflect the radical changes in our knowledge of these tumors, due to a large series of clinicopathological studies carried out over the past few years. Based on a critical review of this growing body of information concerning malignant thyroid neoplasms, a working formulation for clinical usage is proposed. This scheme has been adopted to classify 1339 consecutive cases of thyroid carcinoma observed at the Pathology Institute of the University of Florence between June 1966 and December 1993. The clinical implications of newer classification systems with regard to diagnosis, treatment and outcome of thyroid carcinoma are also discussed.