Smooth muscle antibodies (SMAs) are diagnostic markers for the serological diagnosis of type 1 autoimmune hepatitis. SMA that is restricted to staining of the stomach muscle and blood vessel walls was referred to as "SMA-V". In addition, SMAs are classified into the peritubular (SMA-T) and glomerular (SMA-G) patterns. SMAs are occasionally present in patients with malignancies, but have not yet been reported in thyroid cancer. We came across the first case of SMA positivity in a patient with papillary thyroid carcinoma (PTC). A 31-yr-old male was admitted to our hospital for evaluation of incidentally detected thyroid cancer. He had been diagnosed with PTC based on pathological results following fine-needle aspiration biopsy. The patient underwent total thyroidectomy followed by radio-iodine treatment. The serum levels of AST and ALT were increased before radiotherapy. Tests were conducted for the evaluation of liver disease. SMA was positive at a titer of 1:320, showing positive results for the vessel walls but negative results for the glomerulus and tubules in the kidney (SMA-V pattern). The association of SMA with malignancies and the classification of SMA immunofluorescent subtypes have been previously reported. However, these studies have not clearly established the ability of SMA subtype to predict a specific disease. Therefore, evaluation of an association of SMA pattern with specific diseases in SMA-positive patients may provide additional and useful information for the rapid diagnosis and accurate treatment of patients with autoimmune diseases or malignancies. This case report could serve as a great resource for further studies on SMA.