Cytogenetic analysis of some patients with CTCL demonstrates the presence of more than one cytogenetically aberrant clone in a given patient. These findings lead us to suggest that CTCL is defined by a family of "genotraumatic" T cells. A genotraumatic T cell, unlike a normal T lymphocyte, is defined by its ability to develop clonal, cytogenetically visible chromosome aberrations. Based on this hypothesis, an investigation was performed in detail of cell lines from the plaques and blood of a patient with MF. Several genotraumatic T cells could be demonstrated. Of particular interest was the establishment of two continuous T-cell lines from a single plaque. Both genotraumatic T-cell lines were genetically unstable, and multiple and complex chromosome aberrations could be demonstrated in both cell lines, suggesting that two potentially malignant T-cell clones exist in a single plaque. It is proposed that CTCL is defined by a family of genotraumatic T cells and is thus, in principle, oligoclonal or polyclonal. All genotraumatic T cells may be considered cancer prone because of their ability to develop clonal chromosomal aberrations. A genotraumatic T cell is per se not malignant, but owing to its genetic instability, it may develop into a tumor cell. This could explain how an apparent benign disorder, CTCL, occasionally may progress into malignant lymphoma.