From a cohort of 988 patients with differentiated thyroid carcinoma receiving primary surgical treatment between 1946 and 1970, we studied the 85 (9%) patients who had distant metastases diagnosed during life. Clinically detected metastases were found in 7% of the 859 patients with papillary cancers, 19% of the 100 patients with follicular cancers, and 34% of the 29 patients with Hürthle cell cancers. The total experience amounted to 607 patient-years of observation after the diagnosis of metastases, with a median follow-up in the 12 survivors of 23 yr (range, 13-32 yr). At the time of first diagnosis of metastases, the lungs only were involved in 53%, and bones only in 20%; 16% had multiple organ involvement. The overall mortality rates 5 and 10 yr after the diagnosis of metastases were 65% and 75%, respectively. Seventy-eight percent of all deaths were directly attributable to thyroid cancer; 82% of cancer deaths occurred within 5 yr. By univariate analysis, patient age, tumor extent, pattern of lung involvement, radioiodine uptake of the metastases, and radioiodine treatment were significant prognostic factors. By multivariate analysis, only age (as a continuous variable) at the time of first diagnosis of distant metastases (P less than 0.0001) and involvement of multiple organ sites (P = 0.0003) were independently associated with cancer mortality. The survival at 5 yr in 12 patients aged less than 40 yr with only a single organ involved was 92%. Older patients (aged greater than or equal to 40 yr) with a single metastasis (n = 59) had a lower survival (38% at 5 yr). The highest risk of cancer death (92% at 5 yr) was found in the 14 patients (any age) who at the time of first diagnosis of metastases had multiple organ involvement. The Cox regression model suggested that radioiodine therapy did not have a significant influence on survival, after adjusting for age and extent of metastatic involvement.