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Objectives This study aimed to determine the characteristics and significance of a probable diagnostic entity for primary headache disorder (PHD). Background A diagnosis of probable primary headache disorder (PPHD) is given when a patient's headache fulfills all but one criteria of the third beta edition of the international classification of headache disorder (ICHD-3β). Despite the uncertainty regarding this diagnosis, the inclusion of a probable diagnosis entity in this manual may aid in the accurate classification of headache disorders and allow effective treatment strategies to be started at the patient's initial visit. Methods This cross-sectional multicenter registry study assessed first-visit patients with complaints of headaches who presented at the outpatient clinics of 11 neurologists in Korea. The classification of a headache disorder was made according to the criteria of the ICHD-3 β by each investigator based on the initial evaluation of the patient or by a consensus meeting for uncertain cases. The rates of a probable diagnosis among PPHD patients were assessed and the clinical characteristics of these patients were compared with those of patients with a diagnosis of definite primary headache disorder (DPHD). Results A total of 1429 patients were diagnosed with PHD, and 305 (21.3%) of these patients had PPHD. The proportions of PPHD differed among the subtypes of DPHD as follows: migraines (16.1%), tension-type headaches (TTH; 33%), trigeminal autonomic cephalalgia (TAC; 40.9%), and other PHD (14%, P < .001). Patients with PPHD had less severe headache intensity than patients with DPHD (5.8 ± 2.2 vs 6.5 ± 2.1, respectively, P < .001) as well as a shorter duration of headache from onset (median: 1 vs 4 months, respectively, P < .001). The most common criteria missing for a definite diagnosis in the PPHD patients were total frequency (52.1%), duration of attack (14.4%), and accompanying symptoms (13.1%). Conclusions A probable diagnosis was given to 21.3% of the first-visit PHD patients due to incomplete or atypical presentations of the headaches. The incorporation of a probable diagnosis into the ICHD-3 β may be useful for reducing the diagnoses of unspecified headaches. [ABSTRACT FROM AUTHOR]