Ovarian clear cell carcinoma often shows stromal hyalinization. The main constituents of hyalinization are basement membrane materials, including laminin and type IV collagen. Although it is known that clear cell carcinoma cells produce these materials, it remains unclear whether they can form hyalinized stroma by themselves or if cooperation with stromal cells is required. In the present study, we first reviewed 35 surgical specimens for the pattern of early hyalinization. It occurred either in a globule-like pattern or in a circumferential pattern. In the former, compact hyaline globules abruptly appeared within tumor cell aggregates. In the latter, hyalinized materials appeared around the preceding spherule-like mucoid spaces among tumor cells. In either pattern, hyalinization is most likely to begin in the intercellular spaces among tumor cells, where stromal cells rarely intervene. To verify this, 2 ovarian clear cell carcinoma cell lines (JHOC-5 and HAC-2) were analyzed in vitro. Each cell line was monocultured in suspension: if any deposition occurred in floating multicellular aggregates, it should be in the intercellular spaces. Deposition of type IV collagen occurred in a globule-like pattern (JHOC-5) or a circumferential pattern (HAC-2) within multicellular aggregates, and it developed into a structure comparable with the hyalinized stroma in surgical specimens. Intercellular deposition of type IV collagen was reproduced by culture in 3-dimensional type I collagen gels. All of these findings showed that clear cell carcinoma cells themselves form hyalinized stroma by depositing self-made basement membrane materials in the intercellular spaces.