Background: Mycosis fungoides(MF) is a representative of cutaneous T cell lymphoma and progresses through clinical stages, such as initial pre-mycotic(macule or patch), plaque, and tumor stage. MF is usually thought of as a disease of old adults, but it can develop at any age, including childhood. Despite the recognition that MF may occur in pediatric patients, there is scant 1iterature regarding clinical feature, histopathologic finding and prognosis specially related to childhood onset. The purpose of this study was to examine the clinical, histopathologic and follow up observation of MF in children, Methods: This study has been reviewed in the clinicopathologic findings of 14 MF patients who visited the Kosin University Gospel Hospital during about 10 years from January, 1991 to June, 2000. Results: 1. Duration of cutaneous lesions was from 6 months to 10 years, with mean duration of 3.6 years. 2. The morphology of skin lesions showed various features(pityriasis lichenoides-like, hypopigmented, hyperpigmented, ichthyosis-like, inflammatory linear verrucous epidermal nevus-like). 3. Histopathologic features revealed epidermotropism, a perivascular infiltrate in all cases, haloed lymphocytes, lymphocytes aligned in the basal layer, and coarse collagen in the papillary dermis in most cases. Pautrier's microabscess was shown in 3 cases (23%). 4. In T cell receptor γ gene arrangement using PCR (polymerase chain reaction), monoclonality was detected in 13 out of 14(93%). 5. Treatment including PUVA (psoralen and UVA), UVB, topical steroid agents and calcipotriol had good a response. 6. No patients had progressed to a more advanced disease. Conclusion: MF in children of this study showed a wide variety of skin lesions. Although no case of MF in this study progressed to a more aggressive status, MF in children should be carefully followed for development of more advanced cutaneous lesions and visceral involvement. (Korean J Dermatol 2001; 39(4): 413~419)