Hyaluronan (HA) is a ubiquitous, major component of the pericellular matrix and is necessary for various physiological processes. It plays a very important role in biological barriers. We previously reported that 4-methylumbelliferone (MU) inhibits HA synthesis and pericellular HA matrix formation in cultured human skin fibroblasts, Streptococcus equi FM100, and B16F10 melanoma cells. We hypothesized that MU-mediated inhibition of HA synthesis and pericellular HA matrix formation would increase the efficacy of anticancer drugs. We have already demonstrated in vitro, using a sandwich binding protein assay and a particle exclusion assay, that MU inhibits HA synthesis and formation of the pericellular HA matrix, respectively, in human KP1-NL pancreatic cancer cells. AlamarBlue assay revealed that the anticancer effect of gemcitabine in KP1-NL cells was increased by pretreatment with MU. In vivo simultaneous administration of MU and gemcitabine to tumor-bearing mice with severe combined immunodeficiency disease (SCID) decreased the size of the primary and metastatic tumors more than did gemcitabine alone. These data strongly suggest that a combination of MU and gemcitabine is effective against human pancreatic cancer cells. MU may have potential as a chemosensitizer and may provide us with a new anticancer strategy.