In forensic sciences, identity and bloodline determination are established using DNA technology. This technology developed a lot in the recent years. Interventions such as blood transfusion and bone marrow transplantation for the treatment of various conditions, prior to sample collection may negatively affect test results. Namely, DNA belonging to the blood cells of the individual who underwent bone marrow transplantation is the same with the DNA of the donor. Thus, identity and paternity determination may yield erroneous exclusion when blood is used. Medical histories of the subjects should absolutely be questioned when taking samples. Otherwise, erroneous exclusions may not be avoided. The aim of this study was to call attention to the necessity of knowing medical histories of the subjects in forensic DNA studies.