A total of 263 patients with histopathologically proved, well-differentiated papillary and follicular thyroid carcinoma were treated with sodium iodide I 131 after surgery. A 100% follow-up was achieved with a mean of 6.3 years, and an observation period of 0 to 20 years. The death rate in these patients was compared with the death rate in 50 patients with papillary and follicular carcinoma of the thyroid treated surgically before introduction of sodium iodide I 131 (control group), and followed for a mean of 6.7 years (0 to 20 years). These groups were matched in sex, but not in age distribution. In patients 40 years of age and older, the group treated with sodium iodide 1131 had a significantly lower death rate than the surgery-only group (P<0.005). There was no significant difference in death rate between patients treated with sodium iodide 1131 and control patients 39 years of age and under.