Acquired hemophilia A (AHA) is a rare autoimmune disorder with severe, even life-threatening hemorrhage, resulting from polyclonal antibodies of IgG (IgG) class with affinity to coagulation factor VIII (FVIII).We reported the first case of AHA in a patient with right lung squamous cell carcinoma treated with tislelizumab.A 74-yr-old male patient was diagnosed with right lung squamous cell carcinoma in Feb. 2023.At this time, his disease was at an advanced stage (T1N3M1 stage IV), progressing with multiple metastases (mediastinal, abdominal, retroperitoneal multiple lymph nodes, both lungs, bone, kidneys, and left adrenal gland).He received a first course of treatment with chemotherapy and immunotherapy, with paclitaxel 175 mg/m2, carboplatin 5AUC, and tislelizumab 200 mg, by i.v. infusion every 3 wk.Results suggested the diagnosis of AHA.No hemorrhagic complication was noted.However, at the AHA diagnosis, the patient was at the advanced stage with a terminal condition of cachexia, severe electrolyte disorder, and multiple organ dysfunction, immediately followed by hypovolemic shock.Therefore, treatment of AHA was not feasible.In our case, unfortunately, the patient was diagnosed with AHA when his lung squamous cell carcinoma reached the end stage, and soon multiple organ failures occurred, and the patient lost the opportunity for further treatment.With the increasing utility of PD-1 and PD-L1 inhibitors in the clinic, newly identified side effects such as irAEs that accompanied clin. benefits will continue to raise significant concerns.The case we report here represents a rare irAE, AHA, probably related to tislelizumab treatment.Regrettably, the patient had multiple organ failures, resulting in no feasible further treatment.