Objectives: The aim of this study was to develop and test an online questionnaire which would be used in headache epidemiological research in Estonia.Methods: An online questionnaire consisting of 14 headachetargeting questions was compiled. A specifically designed algorithm using the ICHD-3 beta criteria provided the headache diagnosis. All patients aged 18-65 years who had received a definite headache diagnosis by a specialist at Tartu University Hospital's Headache Clinic from February 2014 to March 2015 were invited to complete the questionnaire. The diagnoses given by the headache specialists were compared to the diagnoses proposed by the algorithm to assess its sensitivity and specificity as well as its positive and negative predictive values. Results:The specificities and sensitivities of the main diagnostic groups were as follows: migraines 0.97 and 0.56, tension-type headaches 0.92 and 0.52, trigeminal autonomic cephalalgias 1 and 0.5, and other primary headaches 0.98 and 0.5 respectively. Incorporating ICHD-3 beta probable criteria in addition to the definite criteria for the migraine and tension-type headache diagnostic groups did not decrease the specificities markedly (0.9 and 0.92 respectively) but the sensitivity increased considerably (to 0.8 and 0.6 respectively). Conclusions:The PRILEVEL questionnaire and algorithm have very high specificity. Strictly applying the ICHD-3 beta definite criteria within the epidemiologic studies can lead to an underestimation of true prevalence values of primary headache disorders, and probable criteria should be included to increase sensitivity and decrease the influence of a recall bias.