Haar NMT, et al. - In a large cohort of patients with undefined systemic autoinflammatory diseases, the clinical characteristics, treatment response, and genetic findings were investigated. From the Eurofever registry, an international web-based registry that retrospectively collects clinical information on patients with autoinflammatory diseases, researchers retrieved data of 187 patients. A chronic disease course was reported in seven patients, a recurrent disease course was reported in 180 patients. Disease onset was reported at the median age of 4.3 years. A median of 12 episodes per year, with a median duration of 4 days, was reported in patients. Arthralgia (n = 113), myalgia (n = 86), abdominal pain (n = 89), fatigue (n = 111), malaise (n = 104) and mucocutaneous manifestations (n = 128) were the most commonly reported symptoms. Affections were identified in relatives of 24 patients. Genetic variants in autoinflammatory genes were evident in 15 patients. Patients with genetic variants vs patients without genetic variants had affected relatives more frequently. Good response to non-steroidal anti-inflammatory drugs (NSAIDs), corticosteroids, colchicine, and anakinra was evident in most patients. NSAIDs alone rarely led to complete remission. Patients with distinctive symptoms displayed notable patterns. Patients with pericarditis (n = 11) vs other patients were older at disease onset (33.8 years) and had fewer episodes per year (3.0/year). Patients with an intellectual impairment (n = 8) were younger at disease onset (2.2 years) and often had relatives affected (28.6%).