Background Childhood cases of cytotoxic T-cell lymphoma have not been well described. Objectives We have undertaken an immunohistochemical and genotypic analysis of patients presenting with juvenile onset mycosis fungoides (MF). Patients/methods Of 10 patients presenting over a 3-year period, six exhibited a CD8-positive phenotype. These six patients were also CD2, CD3 and TIA1 positive, but CD56 negative. Apart from the cytotoxic phenotype, these patients had clinicopathological features that were indistinguishable from ordinary cases of MF, with slowly evolving patches and plaques. Three patients were staged as 1A and three as 1B, with no evidence of nodal or systemic disease. Results Patients responded well to conventional therapy, with no evidence of disease progression after 3 years follow-up. Epidermotropism was a prominent feature in four of the six cytotoxic cases. In two patients with an equivocal histology the diagnosis was confirmed by the finding of a clonal population, using polymerase chain reaction/single strand conformational polymorphism analysis of the T-cell receptor gamma gene in lesional skin. The same technique revealed that all blood samples analysed were polyclonal. Conclusions These data show that cytotoxic T-cell lymphoma can pursue an indolent course and that cases of CD8-positive MF may be over-represented in childhood.