Opsoclonus-myoclonus syndrome is a rare neurological disorder. In children, the etiology varies, although it is a paraneoplastic manifestation (mainly of neuroblastoma) in 40% to 80% of cases. Whole-body MRI promises to be a powerful tool in the search for a possible primary tumor in this condition for which the diagnostic algorithm is yet to be established. We present the case of a two-year-old boy with signs of opsoclonus-myoclonus syndrome in whom a retroperitoneal neuroblastoma was detected by whole-body MRI.