Background: Fibromyalgia (FM) is characterized by chronic widespread pain with a low pain threshold. The aim of this study was to compare two criteria for the diagnosis and assessment of FM and to analyze the correlation and agreement between the 1990 and 2010 American College of Rheumatology (ACR) preliminary diagnostic criteria for FM. Methods: We studied 98 patients who had already been diagnosed as having FM using the 1990 criteria or 2010 preliminary criteria. Tender point examination, FM impact questionnaire (FIQ) and pain visual analog scale (VAS) were obtained. According to the preliminary criteria, FM was quantified as WPI (widespread pain index) and the SS scale (symptom severity) and the two criteria were compared. Results: Among 98 patients, 78.6% of the patients were diagnosed with the 1990 ACR criteria and 93.9% of the patients were diagnosed with the ACR preliminary diagnostic criteria, and there was also significant agreement between the two criteria (P < 0.01). There was a correlation with the WPI and the tender point, with the SS and the FIQ, and with the sum of the WPI and SS and the FIQ. Conclusions: The ACR preliminary diagnostic criteria for FM were in agreement with the 1990 ACR criteria during the disease course. The preliminary criteria were the more sensitive method than the 1990 criteria. In addition, the 2010 criteria might have advantages since it is easy to assess the physical and psychological symptoms and can be quantified. Therefore, the ACR preliminary diagnostic criteria for FM could be used more conveniently for clinical diagnosis and follow up evaluation after starting management of FM.