Mature T-cell malignancies of extracutaneous origin are rare disorders. T-cell prolymphocytic leukaemia (T-PLL) is the most common form of all mature T-cell leukaemias in adults. Secondary skill involvement by T-PLL has been reported in 25% of patients. A case of T-PLL which presented with cutaneous infiltration mimicking a cellulitis-like lesion resistant to antibiotic therapy is described. The diagnosis of T-PLL was subsequently fully supported by the clinical, laboratory and cytological findings, as well as by the immunophenotypic study of the skin biopsy. The present case stresses the importance of accurate evaluation of skin lesions in the diagnosis of some haematological conditions and gives additional information about T-PLL such as a previously non-reported cytogenetic abnormality [t(6;6)] and lack of cutaneous lymphocytic-associated antigen expression.