This case was reported in a literature article and described the occurrence of blastic plasmacytoid dendritic cell neoplasia in a 71-year-old male patient who received Influenza vaccine. On an unknown date, the patient received Influenza vaccine. On an unknown date, 7 days after receiving Influenza vaccine, the patient experienced blastic plasmacytoid dendritic cell neoplasia (serious criteria GSK medically significant), neoplasm skin, injection site erythema, injection site plaque and injection site induration. The patient was treated with diflorasone diacetate and Radiation Therapy. On an unknown date, the outcome of the blastic plasmacytoid dendritic cell neoplasia, neoplasm skin, injection site erythema, injection site plaque and injection site induration were recovered/resolved. The reporter considered the blastic plasmacytoid dendritic cell neoplasia, neoplasm skin, injection site erythema, injection site plaque and injection site induration to be possibly related to Influenza vaccine. Additional information received: This case was reported in a literature article and it described the occurrence of a blastic plasmacytoid dendritic neoplasm in a 71-year-old male patient who had received an unspecified influenza vaccine (manufacturer unknown). The patient did not have any relevant history according to the authors. He did not report any previous reactions to vaccinations. No further details of his medical or family history, concurrent medication or concomitant conditions were provided. On an unspecified date, the patient received an unspecified influenza vaccine (dosage unknown; administration route unspecified; site: left upper arm; batch number: not provided). On an unspecified date, 7 days after the vaccination, the patient presented with a 5.0-cm non-tender, erythematous and scaling plaque with an indurated border at the administration site. The patient did not report any systemic symptoms. The initial diagnosis was injection site hypersensitivity reaction, but this was later changed to blastic plasmacytoid dendritic neoplasm based on the biopsy and CD41, CD561, TCL1, and CD1231 immunoprofiling results. This diagnosis was confirmed by a senior haematopathologist and a major cancer centre, but the authors also commented that this could be a blastic plasmacytoid dendritic neoplasm-like reaction to the vaccination. Atypical dense lymphoid infiltrate in the papillary and reticular dermis with a grenz zone was seen on punch biopsy. The infiltrate showed prominent perivascular distribution and the individual lymphocytes had hyperchromatic nuclei with irregular nuclear contour and nucleoli. A crush artefact was noted at the borders of the sample. Immunohistochemical profiling found the lesion to be diffusely positive for CD41 and CD561 with most of the lesional cells been also positive for CD1231 and TdT1. Repeat CD123 staining found more intense and diffuse positivity. Admixed are rare CD791-positivie B cells. Tests for myeloperoxidase, CD34, MUM1, PAX5, CD3, CD10, CD21, kappa and lambda were negative. Epstein Barr virus in situ hybridization results were also negative. Another hospital found that the lesion was positive for TCL11 and negative for CD117 and muramidase. Initial treatment, prescribed before the diagnosis of neoplasm, consisted of topical diflorasone diacetate 0.05%. Radiation therapy was later delivered at the site. The outcome of the event was resolution as the patient reported that the entire lesion had disappeared with the topical treatment at a 2-week follow up appointment. The authors commented that based on the apparent regression of the lesion and the lack of a specific diagnostic test for this condition, this case could either be a distinct presentation of a blastic plasmacytoid dendritic neoplasm that carries a good prognosis or it could be a blastic plasmacytoid dendritic neoplasm-like reaction to the vaccination. The authors concluded that "in the English-language literature, there are no reports, to our knowledge, of a blastic plasmacytoid dendritic neoplasm occurring at the vaccination site. In addition, a blastic plasmacytoid dendritic neoplasm also carries a poor prognosis, and the regression of our patient's lesion with topical high potency steroid makes our case novel and intriguing. The diagnosis of this lesion is dependent on histological and immunologic evaluation. To date, there is no specific molecular test to confirm a diagnosis of a blastic plasmacytoid dendritic neoplasm. Therefore, based on the current standards, a diagnosis of a blastic plasmacytoid dendritic neoplasm was made on our biopsy. However, the atypical presentation and behavior of our case raises the question if our lesion was indeed a distinct presentation of a blastic plasmacytoid dendritic neoplasm that carries a good prognosis or was a blastic plasmacytoid dendritic neoplasm-like reaction to the vaccination. At the time of this report, the benign clinical course of this case is highly uncharacteristic of blastic plasmacytoid dendritic. Regardless, given the possibility of true BPDN and the prognosis it carries, any such lesion with characteristic histology and immunophenotype should be met with extreme suspicion, and treatment measures appropriate to blastic plasmacytoid dendritic should be considered".