Background. Small, well-differentiated thyroid cancer (tumor <1.5 cm) is frequently dismissed as biologically inconsequential, although varied reports have offered differing experiences. Methods. A total of 382 thyroid cancer patients were reviewed. Of these, 99 patients had tumors that were less than or equal to 1.5 cm. Thirty-five patients in this group with positive nodes, extrathyroidal invasion, or metastatic disease were studied. Results. Thirty-five patients (one-third of the <1.5 cm group) showed other sites of involvement: nodes, 28; lung, 1; muscle, 7; nerve, 5; and bone, 2. Six patients had residual cancer following surgery. Surgery included thyroidectomy and neck dissection as well as orthopedic procedures for metastatic bone disease. Radioiodine ablation was used in 33 patients, external radiation in 5. Thirty-one patients are well without disease, 3 are alive with disease, 1 died of disease. Conclusions. Small, well-differentiated thyroid cancer is infrequently aggressive, but it may be a source for metastatic morbidity and recurrence and can be viewed as potentially lethal. Need for treatment should not be ignored based solely on the size of the tumor. (C) 1995 John Wiley & Sons, Inc.