The association of mycosis fungoides and a primary cutaneous CD30+ lymphoproliferative disorder has been reported and probably represents different clinical aspects of a unique T-cell monoclonal expansion. In this study, 12 patients (6 men and 6 women) presented with lymphomatoid papulosis and mycosis fungoides. A TCRgamma gene rearrangement study was performed by an automated high-resolution PCR fragment analysis method on skin biopsy specimens taken from the different clinical lesions in each patient. An indolent clinical course was observed in the majority of patients. T-cell clonality was identified in 7 of 12 lymphomatoid papulosis lesions (58%) and in 6 skin biopsies of plaque stage mycosis fungoides (50%). In each individual case, where T-cell clonality was detected, both mycosis fungoides and lymphomatoid papulosis specimens exhibited an identical peak pattern by automated high-resolution PCR fragment analysis, confirming a common clonal origin. Only one case showed a clonal TCRgamma rearrangement from the lymphomatoid papulosis lesion, which could not be demonstrated in the mycosis fungoides specimen. The demonstration of an identical clone seems to confirm that both disorders are different clinical manifestations of a unique T-cell monoclonal proliferation. Our results also seem to confirm that the association of mycosis fungoides with a primary cutaneous CD30+ lymphoproliferative disorder usually carries a favourable prognosis.