All forms of cutaneous T-cell lymphoma are rare in children. We describe an 11-year-old girl who had generalized exfoliative erythroderma, intense pruritus, peripheral lymphadenopathy, mycosis cells in the skin and lymph nodes, and Sézary cells in the peripheral blood. Results of a biopsy specimen of involved skin showed changes consistent with mycosis fungoides. A classic case of Sézary syndrome has not previously been reported in childhood or preadolescence.