Between November 1965 and December 1983, 293 patients were treated for Graves' disease using 131I. All patients were asked to identify a stressful event antedating the onset of overt clinical symptoms. Eighty-one patients were able to do this (27.6%). Six patients were lost to follow-up, the others were followed from 1 to 155 mo. Two hundred forty-four patients received a single treatment, 49 required two or more treatments. Stress and nonstress individuals were consistent with respect to age, sex, number of treatments and the dose of radioiodine. Patients with stress initiating the symptoms of Graves' disease became hypothyroid earlier, 50% at 12 mo compared with 36 mo for the nonstress group, p = 0.01. At 10 yr 5% of the stress group remained euthyroid compared with 17% nonstress. We conclude that stress in the 12 mo or less before the onset of clinical symptoms potentiates the development of hypothyroidism induced by a standard dose of radioiodine.