Piriform sinus fistula is a rare congenital anomaly and is known to be a route of infection to the thyroid. We report a case of acute suppurative thyroiditis in a 7-year-old girl. Contrast x-ray films of the esophagus revealed a fistula originating from the apex of the left piriform sinus. This fistula was completely removed by surgery which included hemithyroidectomy under general anesthesia. The fistula ran from the left piriform sinus to the left lobe of the thyroid gland. The lumen of the fistula was lined with stratified squamous epithelium. In cases of acute suppurative thyroiditis or repeated cervical infection due to unknown causes, an esophagogram is necessary to determine a route of infection in the piriform sinus fistula.