Objective: To analyze the range of demographic, clinical, MRI, and CSF features of acute disseminated encephalomyelitis (ADEM), a rare, typically monophasic demyelinating disorder, and analyze long-term outcomes including time and risk factors for subsequent clinical events as well as competing diagnoses. Methods: We performed a retrospective, multicenter study in 4 US academic medical centers of all patients clinically diagnosed with ADEM. Initial presentation of pediatric and adult ADEM and monophasic and multiphasic disease were compared. The Aalen-Johansen estimator was used to produce estimates of the probability of transitioning to a multiphasic diagnosis as a function of time since initial diagnosis, treating death and alternative diagnoses as competing risks. Results: Of 228 patients (122 children, age range 1-72 years, 106 male, median follow-up 24 months [25th-75th percentile 6-67], 7 deaths), approximately one quarter (n 5 55, 24%) experienced at least one relapse. Relapsing disease in children was more often diagnosed as multiphasic ADEM than in adults (58% vs 21%, p 5 0.007), in whom MS was diagnosed more often. Encephalopathy at initial presentation (hazard ratio [HR] 0.383, p 5 0.001), male sex (HR 0.394, p 5 0.002), and increasing age at onset (HR 0.984, p 5 0.035) were independently associated with a longer time to a demyelinating disease relapse in a multivariable model. In 17 patients, diagnoses other than demyelinating disease were concluded in long-term follow-up. Conclusions: Relapsing disease after ADEM is fairly common and associated with a few potentially predictive features at initial presentation. Age-specific guidelines for ADEM diagnosis and treatment may be valuable, and vigilance for other, mostly rare, diseases is imperative.