Immune checkpoint inhibitors (ICIs) have led to a revolution in cancer management, mainly due to lasting long-term durable responses in a subset of patients with metastatic solid tumours (Gettinger et al. in JCO 36(17):1675-1684, 2018). As immunotherapy is gradually being applied for the treatment of a large range of solid tumours, the incidence of neurological immune-related adverse events (irAEs) has increased (2). Neurologic toxicities that result in high morbidity rates and even mortality have emerged as serious complications of ICIs (Johnson et al. in J Immuno Cancer 7(1):134, 2019; Wang et al. in JAMA Oncol 4(12):1721, 2018). Small-cell lung cancer (SCLC) is common cause of neurologic paraneoplastic syndrome (Sebastian et al. in J Thorac Oncol 14(11):1878-1880, 2019). Nevertheless, the distinction between neurologic iRAEs and paraneoplastic neurological syndromes (PNSs) in patients with SCLC treated by ICIs remains challenging (Williams et al. JAMA Neurol 73(8):928, 2016). As immunotherapy is gradually being applied for the treatment of a large range of solid tumours, the incidence of neurological autoimmune adverse events has increased. Neurologic toxicities that result in high morbidity rates and even mortality have emerged as serious complications of ICIs and have yet to be fully understood. We report a case of an immune induced cerebellar ataxia in a 47 year-old small-cell neuroendocrine carcinoma patient undergoing checkpoint blockade by atezolizumab, a programmed cell death-1 ligand (PDL-1) inhibitor. After 4 cycles of immunotherapy, the patient presented with kinetic and static cerebellar syndrome leading to the diagnosis of TRIM9-Abs ICI-related cerebellar irAE. Therapeutic management was discussed in multidisciplinary meetings in the lack of therapeutic guidelines. There was no clinical improvement. Because of high morbidity and no treatment evidence, neurologic symptoms developing under ICI require early diagnosis and may indicate the need for definitive treatment discontinuation.