The aim of this work was twofold: to evaluate the prevalence of different Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) diagnosis in an Italian population of subjects seeking TMD treatment in a tertiary clinic; and to compare data with those from similar studies in the literature. Participants in this study were 433 consecutive patients seeking TMD treatment at the Section of Prosthetic Dentistry, Department of Neuroscience, University of Pisa, Italy; mean age of patients was 38·8 years, with a female:male ratio of 2·6:1 (276 females, 73·2%; 101 males, 26·8%). RDC/TMD guidelines for examination were adopted to assign axis I diagnosis. The prevalence of RDC/TMD diagnoses was 38·2% (144/377) for group I disorders (muscle disorders), 52·3% (197/377) for group II disorders (disc displacements), and 52·6% (198/377) for group III disorders (arthralgia, osteoarthritis, osteoarthrosis). The present investigation provided findings that, compared and integrated with literature data, can be useful to create a world-wide database, in accordance with the nature of the RDC/TMD classification system.