OBJECTIVE: To explore the characteristics of symptom flares, individual experiences and behaviours during flares in people with endometriosis. DESIGN: Online questionnaire shared on patient support sites. SETTING: People with a confirmed or working diagnosis of endometriosis (a working diagnosis is given by clinicians based on symptoms/history; individuals may or may not go on to have further imaging/surgical investigations). POPULATION OR SAMPLE: A total of 236 responses were collected. METHODS: Descriptive and comparative analysis of quantitative data, and thematic analysis of qualitative data. MAIN OUTCOME MEASURES: The characteristics, triggers, treatments and strategies for symptom flares together with perceived predictability and self-efficacy in relation to flares, healthcare access during flare, advice received and overall endometriosis-related quality-of-life. RESULTS: We identified a wide variation in the characteristics and treatments/strategies. 31.2% stated that they were 'not at all' confident coping with long flares, and around 1/3 of participants found flares 'not at all' predictable. Only 35.3% reported receiving advice from a healthcare provider about flares. We developed 5 themes to suggest why participants did not contact healthcare providers: 'what can they do?', 'I can cope, it will end', 'broken healthcare system', 'perceived dismissal and gaslighting' and 'symptoms stop me'. CONCLUSIONS: Flares have a large impact on quality-of-life and are clinically very important. Individuals do not commonly receive advice from healthcare providers or contact healthcare providers during a flare. More research, in a more diverse sample, is needed to identify mechanisms underlying flares, as well as developing and disseminating management tools to prevent, manage and treat flares.