Eighty consecutive patients with typical subacute thyroiditis were evaluated. Sex distribution showed a higher incidence in females (F/M 3.2/1), with a mean age of 44 yr. In the majority of patients (51/80 = 66%) the onset of the disease was between June and September (46% in July and August). The remaining cases were distributed in the other months without a clear monthly prevalence. These results indicate that the seasonal distribution of subacute thyroiditis is almost superimposable to that of established infections due to some enteroviruses (Echovirus, Coxsackie A and B viruses), suggesting that summer enterovirus infections may be responsible for a large proportion of cases of subacute thyroiditis.