Sezary cells are malignant T lymphocytes (mostly CD4+, CD8-) seen in Sézary syndrome, a form of cutaneous T-cell lymphoma (CTCL). This disease manifests clinically in the form of a generalized, red rash.

Aberrant lymphocytes have a characteristic irregular nucleus, with parts of the nucleus folded over each other, similar to those of promonocytes. The folding can be seen in the form of thin lines running through the nucleus.