Background: Informal dementia caregivers (IDCs) are often confronted with important fluctuations in care-related burden, commonly described as "good and bad days." These fluctuations are overlooked by traditional questionnaires focusing on the average experience. The experience sampling method (ESM) is based on the repeated collection of data in everyday life, thereby allowing the description of day-to-day fluctuations in IDC burden, and the identification of their correlates. ESM studies are still scarce among IDCs, with none focusing on day-to-day fluctuations in burden.; Objectives: This ESM study aimed to examine day-to-day fluctuations in the burden of IDCs and test their associations with six moment-to-moment predictors.; Methods: Primary IDCs (N = 26, median age = 68 years, 77% women, 73% spouses) volunteered to answer questions about their daily burdens, patients' memories and behavioral problems (MBP), caregivers' MBP-related distress, psychological distress, self-efficacy and positive affects, and relationship quality; volunteers did this every evening for 2 weeks on a touchpad, resulting in 206 measures. Data were analyzed with multilevel linear regression.; Results: Day-to-day fluctuations covered about two thirds of the total variance for most study variables. All six predictors had a significant bivariate relation with daily burden, explaining 15%-32% of its fluctuations, with significant differences between caregivers in the strength of these relations. The best multivariate model explained 51% of the day-to-day fluctuations in burden. It included caregiver MBP-related distress, psychological distress, and relationship quality.; Discussion: This innovative study of IDC burden shows that day-to-day fluctuations are an important part of caregivers' real-life experiences and that half of this variability is predicted by currently understudied factors. Inviting caregivers to monitor clinical outcomes daily over 1 or 2 weeks could help tailor interventions to their individual needs and also empower them.