Summary Cancer is one of the main causes of mortality worldwide. Thanks to scientific research, new methods of cancer treatment, including molecularly targeted therapy, are being developed. Monoclonal antibodies are used to treat many diseases, including some types of cancer, and affect various systems of the human body. The presented article aims to present the adverse effects of molecularly targeted cancer therapy on the endocrine system based on the current literature data. Immune checkpoint inhibitors, such as anti-CTLA-4 and anti-PD-1 or its ligand PD-L1, can cause a variety of autoimmune adverse effects, among others, thyroid dys-function, hypophysitis, and diabetes mellitus. The authors also paid attention to monitoring selected diagnostic parameters to prevent endocrine adverse effects during a therapy with monoclonal antibodies. The development of adverse effects may sometimes progress atypically and rapidly, and may be a life-threatening condition. Clinicians should choose individual schemes of treatment for particular patients. The patient’s condition should also be monitored before, during and after the therapy. The decision about the continuation of treatment with monoclonal antibodies should be based especially on a risk connected with the cessation of treatment. Clinical trials should be continued to improve knowledge about the side effects of monoclonal antibodies.