Teratoma contains elements derived from all three embryonic germ layers and mature teratomas are entirely composed of mature differentiated tissues, while immature types always contain additional embryonic tissues of variable degree of immaturity, especially neuroepithelial elements. Twenty cases of teratoma in infancy and childhood were reviewed and the clinical features and pathologic findings including PCNA expression were studied. Sacrococcygeal teratomas were the most common type(45%), followed by retroperitoneal and ovarian teratomas. There was a predilection of females in a ratio of 4:1 and age distribution was most prevalent below the first year of age(45%). But in ovarian teratomas, the age at diagnosis was above 4 years of age in all cases. Serum alpha-fetoprotein levels were checked in 18 cases. In all mature teratomas and 1 of 5 immature teratomas, the levels were normal. But in 4 of 5 immature teratomas, the serum levels were elevated and progressively declined to normal range after mass excision. Radiologically, calcifications in tumor were found in 60.0% of teratomas and was higher in mature teratomas(69.2%) than immature teratomas(42.9%). Immunohistochemical staining for PCNA(proliferating cell nuclear antigen) was done in 16 cases and PCNA expression was higher in grade III immature teratomas than grade I and II. The operative modes were complete mass excisions. Five immature teratomas were treated with multiagent PEB(Bleomycin, Etoposide, Cisplatin) adjuvant chemotherapy, 3 tolerated well without significant complications, but in one case, severe bone marrow suppression was developed and died of sepsis. In conclusion, grade III immature teratoma showed higher PCNA expression than mature or lower grade immature teratoma, which suggests that chemotherapy after surgical excision may be effective modality for grade III immature teratoma. We think, however, multicenter study is necessary because of low incidence of immature teratoma.