Introduction: Ataxia is a lack of coordination of movement and balance (1). Etiologies range from benign age-related clumsiness or viral cerebellitis to life-threatening malignancies. This case emphasizes the importance of maintaining a broad differential when evaluating ataxia in pediatric patients. Case Report: A previously healthy 14-month-old female presented with sudden-onset ataxia and two recent falls without loss of consciousness. Unsteady gait began four days prior to presentation. She was born full-term without any significant complications, and had normal growth and development. She was discharged with a suspected diagnosis of post-viral cerebellar ataxia and recommendations for supportive care. She returned eight days later with worsening truncal ataxia, new onset esotropia, and horizontal nystagmus. During her first admission, she demonstrated an ataxic gait requiring significant support, foot drag with ambulation, and no apparent nystagmus. Her abdomen was soft and non-tender without palpable mass. During her second admission, she had pronounced truncal ataxia, refusal to walk, and unsteadiness of the entire trunk and head. There was left esotropia and intermittent horizontal nystagmus. She had normal tone, strength, and muscle bulk throughout, without clonus. She continued to have a soft, non-tender abdomen without masses. She had erythematous macules on the lower extremities. Discussion: Differential diagnosis of acute ataxia included acute cerebellar ataxia, intracranial lesion, drug intoxication, post-concussive syndrome, and opsoclonus myoclonus syndrome (OMS), with concern for neuroblastoma. CT head, MRI brain, cervical spine, thoracic spine and lumbar spine were all unremarkable. Blood and CSF cultures negative. Urine and serum drug screen negative. CT chest/abdomen/ pelvis unremarkable. Urine spot check VMA/HVA negative. Oligoclonal bands were present in CSF and serum. CSF autoimmune encephalitis panel was negative. This child’s presentation was initially concerning for acute cerebellar ataxia, possibly secondary to Coxsackievirus. Her lower extremity erythematous macular rash and horizontal nystagmus were suspected to be manifestations of viral labyrinthitis caused by Cocksackievirus. Typically, nystagmus with OMS, called “saccadomania”, has both vertical and horizontal components. Imaging studies failed to reveal evidence of neuroblastoma, further lowering the suspicion for OMS. Ultimately, without the clinical improvement one would expect with acute cerebellar ataxia, and with the presence of oligoclonal bands in CSF, this patient was diagnosed with OMS without neuroblastoma. She likely had intermittent saccadic intrusions, which is atypical with OMS. She was treated with two rounds of IVIG with mild improvement of symptoms. After transitioning to IV rituximab, IVIG and dexamethasone, she had complete resolution of her symptoms and resumption of normal development. Conclusion: It is important to consider the broad differential diagnosis when evaluating a patient with sudden-onset-ataxia. Evaluation of ataxia requires a thorough history and a complete neurologic exam, developmental history and workup. Although post-viral acute cerebellitis was initially suspected, her progressive symptoms suggested an alternative diagnosis.