We describe a case of a patient hospitalized in haematology unit for treatment to blastic plasmacytoid dendritic cell neoplasm. Apart skin lesions found at diagnosis in 83% of patients, few elements suggest the diagnosis. Cytology is not characteristic and no cytogenetic abnormality is specific to the LpDC, the karyotype shows generally at least three cytogenetic abnormalities. Definitive diagnosis rests to identification of a blastic population with immunophenotype CD4+ CD56+. This leukemia is chemosensitive but the relapse rate is important and the survival time is 16 months.