Cognitive and acquired neurodevelopmental deficits have been reported in children with opsoclonus–myoclonus syndrome (OMS) and are known to be associated with more severe and relapsing disease course. However, there is a paucity of data regarding cognitive dysfunction in children with stable neurological disease. We report three children with OMS and evolving cognitive dysfunction in the context of a mild disease course. The children’s ages at disease onset were between 17 and 35 months and they were followed up for 4–10 years. Neuroblastoma was identified in one child. OMS severity scores ranged between 8 and 12/15 at presentation. They underwent immunotherapy and all were in remission by 7 months (range 4–13 months), with treatment maintained for 1 year. One child remained relapse-free, while two others had one clinical relapse each and were immunotherapy-responsive again. In all cases, evolving cognitive dysfunction was reported despite being in remission and stable off treatment for a median of 20 months (range of 12–31 months; two OMS scores of 0/15 and one of 2/15). In children with OMS who have completed treatment and have made full or near full neurological recovery, concerns remain regarding long-term outcome in terms of future learning and cognitive development.