The records of patients treated at the Mayo Clinic for toxic multinodular goiter during 1950–1954, 1960–1964, and 1970–1974 were reviewed to compare results of surgical and131I therapy, to define trends in treatment policy and outcome, and to attempt to elucidate factors associated with posttreatment hypothyroidism. The series involved 446 patients who underwent thyroidectomy as initial treatment and 89 who initially received131I. The time to success was significantly shorter (p=0.001) in surgically treated patients than in patients initially receiving131I. Compared with patients treated medically, patients treated initially with surgery were much less likely to require a second treatment (0% versus 24%) during the first year after initial treatment. The probability of postthyroidectomy hypothyroidism increased from approximately 12% in the 1950s to more than 70% in the 1970s at 2 years after surgery. Neither more aggressive surgery nor use of more sensitive tests to diagnose hypothyroidism accounted for the increased probability in recent years.