We have previously reported that an internal fistula (piriform sinus fistula) was the apparent route of infection in 15 patients with acute suppurative thyroiditis (AST). Here, we describe 43 patients with AST, most of whom had a demonstrable fistula. The characteristic clinical features included: onset in infancy or childhood in 74% of cases, a left-sided predominance of involvement (40:3), and frequent recurrence. Twenty-nine patients had had several previous episodes of AST. The fistula was demonstrable in 38 of 42 patients examined by barium meal. Twenty-seven patients underwent fistulectomy. Six of 16 patients who declined fistulectomy had recurrences and 2 of them underwent surgery thereafter. None of 29 patients who had fistulectomy developed recurrences, except for 2 patients in whom the fistula could not be removed completely. The fistula ended medial or lateral to the thyroid lobe, attached to or entered the lobe in 6, 3, 6, and 12 cases, respectively. The thyroid specimens showed several features of inflammatory change. Thus, we concluded that the piriform sinus fistula is the most common underlying abnormality in patients with AST. Recurrence of inflammation can be prevented by complete fistulectomy.