Alternative splicing in the cyclin D1 gene produces cyclin D1b variant which lacks a C-terminal region containing the threonine-286 (T286) phosphorylation site required for nuclear export. We have shown that the expression of the cyclin D1b variant is detected in about 60% of human bladder cancer tissues (15/26) and cell lines (3/5). To examine the role of the cyclin D1b variant in bladder carcinogenesis, we introduced wild-type cyclin D1a, cyclin D1b variant or mutant cyclin D1-T286A cDNAs into a human bladder cancer cell line, SBT991, in which cyclin D1b transcript was not expressed, and compared their oncogenic activities. Ectopic expression of cyclin D1b promoted cell invasiveness and anchorage-independent growth of the cancer cells. On the other hand, cyclin D1-T286A enhanced anchorage-independent growth, but did not promote cell invasiveness. The amount of nuclear-localized cyclin D1b and cyclin D1-T286A was higher than that of nuclear-localized cyclin D1a. In addition, introduction of siRNA specific for cyclin D1b into cells of the T24 bladder cancer cell line, in which cyclin D1b transcript was expressed, significantly suppressed cell invasiveness. Immunoprecipitation analysis revealed that cyclin D1a and cyclin D1-T286A could bind to cyclin-dependent kinase 4 (CDK4) but cyclin D1b has lost its capacity to associate with CDK4. Unlike cyclin D1a and cyclin D1-T286A, expression of cyclin D1b did not enhance phosphorylation of Rb protein in SBT991 cells. These results indicate that cyclin D1b promotes cell invasiveness independent of binding to CDK4 to enhance Rb phosphorylation.