In this study, relevant cases were retrospectively reviewed to identify patients who were diagnosed as suffering from myofascial pain only according to the research diagnostic criteria/temporomandibular disorders (RDC/TMD) criteria, in order to examine whether or not they could fulfil the American Academy of Orofacial Pain (AAOP) diagnostic criteria for TMD-related masticatory muscle disorders. One hundred and twenty-seven patients, diagnosed according to the RDC/TMD criteria as having myofascial pain with or without limited jaw opening only, were allocated to two groups according to their answers to the RDC/TMD 'jaw disability checklist'. The two groups were compared for Axis I and II data taken from the RDC/TMD questionnaire. Thirty-eight of the patients (29·9%) did not associate their symptoms with jaw functions (e.g. chewing and yawning). This group was characterised by increased range of motion (ROM) and older average age. The AAOP diagnostic criteria for TMD require pain on function in all subtypes of TMD-related muscle disorders. An association between pain and jaw function is common and research is needed to determine whether this should be categorised differently to muscle pain unrelated to jaw function. There may well be different pathophysiological mechanisms and consequently different management strategies for these two pain conditions.