Stevens-Johnson syndrome is a severe delayed type systemic allergic reaction which affects the skin and mucous membranes. In adults, Stevens-Johnson syndrome is usually caused by the administration of drugs or a malignant process. The paper presents a case of Stevens-Johnson syndrome after the treatment of a female patient with small lymphocytic B-cell lymphoma, autoimmune hemolytic anemia and antiphospholipid antibody syndrome with rituximab. A rare combination of Stevens-Johnson syndrome and small lymphocytic B-cell lymphoma of small lymphocytes, as well as the development of severe delayed type systemic allergic reaction to introduction of rituximab are of special interest. A detailed medical history and the clinical manifestations of the disease allowed to diagnose Stevens-Johnson syndrome at early stages and prescribe an adequate therapy. As a result of the treatment, the patient’s condition has improved considerably. Symptoms of general toxicity were arrested completely; there was a complete epithelization of erosive defects. Therefore, the presented clinical observation shows that timely diagnosis, complex drug therapy, and comprehensive care can cure the diseases as soon as possible and prevent complications.