Cutaneous T-cell lymphoma (CTCL) is a rare disease occurring in Europe among two persons per million per year. It affects men more often than women (2:1). It is primarily a skin disease. In about 20% of patients, it becomes fatal with tumours in the skin and spreading to lymph glands. Approximately 3% of patients show a leukemic form called Sezary's syndrome, where malignant cells are present in blood with accompanying erythrodermia. CTCL is a T-lymphocyte disease occurring late in life as the average age of patients is around 66 years in Europe, Japan and the US. This article focuses on cell lines and immune surveillance in CTCL, and especially the pronounced chromosomal instability. It leads to the hypothesis that chromosomal changes is the key event linked to DNA repair deficiencies, which in a subpopulation of T cells leads to CTCL development over years.