A 34-year-old woman was referred to our hospital complaining of sore throat and arthralgia. She had low-grade fever, tachycardia, and goiter with tenderness. Laboratory data revealed thyrotoxicosis and tests for acute inflammatory markers were positive. Thyroidal radioactive iodine uptake was below normal. Ultrasonography of thyroid revealed mild thyroid enlargement and hypoechogenic areas consistent with tenderness. Subacute thyroiditis was diagnosed and prednisone was administered. Two years later, her identical twin sister, who lives separately, was referred to our hospital because of neck pain, low-grade fever, and palpitation. She exhibited the same clinical picture as her twin sister, and was also diagnosed as having subacute thyroiditis. Although the cause of subacute thyroiditis remains unclear, viral infection has been implicated in the onset of subacute thyroiditis in genetically predisposed individuals. We could not identify the viruses, but heterozygotes for HLA-B35, which has been reported to be linked with subacute thyroiditis, were found in the twins. This supports the suspicion that genetic factors, including this HLA haplotype, play a critical role in the onset of subacute thyroiditis.