ABSTRACT: OBJECTIVE:To evaluate factors associated naturalistically with adherence to a mobile headache diary. BACKGROUND:Self-monitoring (keeping a headache diary) is commonly used in headache to enhance diagnostic accuracy and evaluate the effectiveness of headache therapies. Mobile applications are increasingly used to facilitate keeping a headache diary. Little is known about the factors associated with adherence to mobile headache diaries. METHODS:In this naturalistic longitudinal cohort study, people with headache (n = 1561) registered to use Curelator Headache® (now called N1-Headache®), an application that includes a mobile headache diary, through their physician (coupon), or directly through the website or app store using either a paid or free version of the application. Participants completed baseline questionnaires and were asked to complete daily recordings of headache symptoms and other factors for at least 90 days. Baseline questionnaires included headache characteristics and migraine disability. Daily recordings included headache symptoms and anxiety ratings. Adherence to keeping the headache diary was conceptualized as completion (kept the headache diary for 90 days), adherence rate (proportion of diary days completed 90 days after registration), and completion delay (the number of days past 90 days after registration required to complete 90 days of headache diary). RESULTS:The majority of participants reported migraine as the most common headache type (90.0%), and reported an average of 30.8 headache days/90 days (SD = 24.2). One-third of participants completed 90 days of headache diary (32.4%). Endorsing higher daily anxiety scores (8/10 OR = 0.97 [95% CI = 0.96, 0.99]; 10/10 OR = 0.96 [95% CI = 0.91, 0.99]) was associated with lower odds of completion, whereas higher age (OR = 1.04 [95% CI = 1.03, 1.05]), and downloading the app paid vs free (OR = 4.27 [95% CI = 2.62, 7.06]), paid vs coupon (OR = 2.43, 95% CI = 1.41, 4.26]), or through a physician coupon vs free (OR = 1.75 [95% CI = 1.27, 2.42]) were associated with higher odds of completion. The median adherence rate at 90 days was 0.34 (IQR = 0.10-0.88), indicating that half of participants kept 34 or fewer days 90 diary days after registration. Endorsing high daily anxiety scores (5/10 OR = 0.98 [95% CI = 0.97, 1.00]; 8/10 OR = 0.96 [95% CI = 0.94, 0.98]; 10/10 OR = 0.96 [9% CI = 0.92, 0.98]) and higher age (OR = 1.05 [95% CI = 1.04, 1.07]) were associated with lower odds of adhering at 90 days, whereas downloading the app paid vs free (OR = 9.63 [95% CI = 4.61, 25.51]), paid vs coupon (OR = 2.39, 95% CI = 1.27, 5.10]), or through a physician coupon vs free (OR = 4.01 [95% CI = 2.54, 7.26]) were associated with higher odds of adhering at 90 days. Among completers, the median completion delay was 6.0 days (IQR = 2.0-15.0). Among completers, endorsing high daily anxiety scores (9/10 OR = 1/06 [95% CI = 1.01, 1.12]) and younger age (OR = 0.98 [95% CI = 0.97, 1.00]) was associated with completion delay; downloading the app through physician coupon vs free (OR = 0.40 [95% CI = 0.22, 0.71]) or paid vs free (OR = 0.38 [95% CI = 0.20, 0.72]) was associated with lower odds of completing 90 diary days in 90 calendar days. CONCLUSION:This naturalistic observational study confirmed evidence from clinical observation and research: adherence to mobile headache diaries is a challenge for a significant proportion of people with headache. Endorsing higher levels of daily anxiety, younger age, and downloading the app for free (vs either paying for the self-monitoring app or receiving a physician referral coupon) were associated with poorer adherence to keeping a mobile headache diary.