The histopathological features of hepatoblastoma in 17 patients treated with preoperative chemotherapy were compared with those in 11 patients not subjected to chemotherapy during the same 11-year period. Tumor necrosis was more extensive in patients receiving preoperative chemotherapy. Two tumors, however, were apparently unaffected by chemotherapy. There was no obvious correlation between the extent of necrosis and the number of courses of chemotherapy. There also seems to be no evidence of preferential ablation of a particular morphological type of tumor. The most notable feature in cases treated with chemotherapy was the extensive presence of osteoid. Osteoid was present in 36% of untreated cases, occupying < 5% of the surface area, compared with 82% in the treated group. In seven cases, osteoid occupied > 40% of the surface area. This finding raises speculation about the role of chemotherapy in the maturation of tumors that have an inherent ability to differentiate. A long-term study is needed to clarify the prognostic significance of mature heterologous elements in hepatoblastoma.