A forensic pathological case of rare T-cell leukemia is reported. Autopsy samples (paraffine blocks and slides) from 32 years old female with long-term existing anemia and leukopenia, number of chronic inflammatory diseases, died from fulminant pneumonia, were presented for forensic pathological re-examination. A routine histological examination was done using standard H&E stain. Additionally, we used IHC stain with antibodies to CD3 and CD20 when examining bone marrow and liver samples. The T-cell infiltration in bone marrow (interstitial and intrasinusoidal type) were found. The next step was IHC bone marrow examination with CD4, CD5, CD8, CD56 antibodies. We concluded that the patient had unrecognized T-cell leukemia, based on anamnestical, clinical data and results of our histological and IHC examination. In this case, the T-cell leukemia was not found neither vitally nor at primary forensic pathological examination after autopsy. The necessity of minimal IHC stain sets usage in primary forensic pathology units and IHC’s decisive role in bone marrow pathological examination are discussed.