Opsoclonus-myoclonus-ataxia syndrome (OMAS) due to Pembrolizumab: a case report Background Opsoclonus-myoclonus-ataxia syndrome (OMAS) is a rare yet debilitating neurological condition in adults; few reports have documented this condition in relation to immune checkpoint inhibitors (ICIs). Pembrolizumab, an ICI, used in the treatment of many forms of cancers can cause a wide range of side effects referred to as immune-related adverse events (IRAEs). Case We report a case of a patient with breast cancer who developed OMAS while receiving neoadjuvant chemotherapy that included pembrolizumab. Extensive work-up was unrevealing. She received steroids for treatment of OMAS that were discontinued due to side effects. Discussion ICI toxicity needs to be considered on the differential for patients with new-onset OMAS. Paraneoplastic panels play an important role in determining the etiology of OMAS. Establishing an etiology for OMAS is important, especially in patients with underlying malignancy, due to the impact on the treatment of OMAS and the malignancy.