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Background: During the last two years, the COVID-19 pandemic led to millions of disease-related deaths worldwide. The efforts of the scientific community facing this global challenge resulted in outstanding achievements. Thus, within one year, new mRNA-based vaccines against SARS-CoV-2 viral infection were released, providing highly efficient protection and showing a very good safety profile in the general population. However, clinical data collection after vaccination is a continuous process for the long-term safety of any new medical product. The aim of our paper is to present two cases of hematological malignancies: diffuse large B-cell non-Hodgkin lymphoma and T/NK-cell lymphoma, diagnosed shortly after the administration of the mRNA COVID-19 vaccine. Methods and Results: Case 1: A female patient was admitted with a suspicious cervical mass that emerged within one week after the administration of second dose of the BNT162b2 COVID-19 vaccine. Surgical removal followed by pathology assessment of the specimen confirmed the diagnosis of diffuse large B-cell non-Hodgkin lymphoma. Case 2: A male patient was admitted with multiple ulcerative oral lesions arising on the third day after the initial dose of the BNT162b2 COVID-19 vaccine. These lesions had a progressive character and during the following months were complicated with repetitive episodes of heavy oral bleeding, requiring blood transfusions. The incisional biopsy of the lesions and pathological assessment of the specimens confirmed the diagnosis of T/NK-cell lymphoma. Conclusions: The safety profile of the mRNA-based vaccines is an undeniable fact. In most cases, suspicions of potentially aggressive side effects were ruled out, proving to be transient post-vaccine reactions. Clinicians should remain alert to report any potentially aggressive manifestations emerging in the context of mRNA COVID-19 vaccination, such as these cases of hematological malignancies, in order to promote additional investigations on the particular mechanisms of action of COVID-19 vaccines and to provide the best medical care to the patients.