Study on the biological activity of compounds consisting only of the C/D ring of vitamin D has been very limited. We found that a synthetic C/D ring alcohol 3 reduced the viability of HeLa cells. Compound 3 was more active than vitamin D2 (VD2) against solid cancer cell lines HeLa and A549 (1 day incubation) and was a little weaker than VD2 against blood cancer cell lines KMS-12-PE and MT-2. Compound 3 rapidly induced cell killing in A549 cells. This compound was compared with staurosporine and it seemed to have a different mechanism of cell killing. Compound 3 did not show clear cytotoxicity to confluent cells that have terminated proliferation, indicating that the activity can be observed only when cells are proliferating. Furthermore, 3 induced cell death, possibly cytotoxicity, in A549 cells (at least 64%). This is the first example of vitamin D C/D ring derivative that kills cancer cells.