Adult onset Still's disease (AOSD) is an inflammatory disease of unknown etiology, characterized by spiking fever, evanescent salmon pink maculopapular rash, arthritis, and leukocytosis with neutrophilia. Malignant lymphoma is one of the most important differential diagnoses of AOSD. AOSD has been reported as one of paraneoplastic syndromes associated with breast cancer. We report a rare case of occult papillary thyroid cancer (PTC) diagnosed coincidently with AOSD. A 32-year-old woman was diagnosed with AOSD according to the diagnostic criteria of Yamaguchi as follows: leukocytosis with neutrophilia, high fever with 39 degrees C and above, arthralgia/arthritis, sore throat, liver dysfunctions, and lymphadenopathy. Excisional biopsy of cervical lymph node showed metastatic papillary carcinoma, and immunohistochemical staining for thyroglobulin and thyroid transcription factor-1 was strongly positive. There was no evidence of focal lesion in the thyroid glands. To our knowledge, this is the first report of adult onset Still's disease diagnosed concomitantly with occult PTC.