Dysregulated CD44 expression is a feature of most human cancers, including prostate cancer (PCa). PCa loses expression of CD44 standard (CD44s) which is present in benign epithelium, and overexpresses a novel splice variant isoform, CD44v7-10, specifically facilitating fibronectin binding and invasion. Naturally-occurring or synthetic phenyl-methylene hydantoin (PMH) and S-ethyl PMH (S-PMH) can reportedly augment cell-cell adhesion, and reduce invasion and growth of PCa. Benign BPH-1 and malignant PC-3M prostate cells were treated with PMH or S-PMH for 36 h and cells were harvested. Cell adhesion assays were carried out. Cancer cells' expression of total CD44 and CD44v7-10 were tested by western blot analysis and real-time RT-PCR. Compared to BPH-1 or PC-3M cells treated with vehicle only, PMH-or S-PMH-treated benign and malignant cells had decreased adhesion to hyaluronan (p=0.001 to 0.007) and fibronectin (p<0.001 to 0.047). Both compounds decreased PCa expression of CD44 total mRNA (representing mostly CD44s, to 0.076+/-0.033 and 0.254+/-0.123 of control) and CD44v7-10 (to 0.386+/-0.279 and 0.115+/-0.037 of control). S-PMH but not PMH decreased CD44 total protein, while both decreased CD44v7-10 protein. Both hydantoins lowered beta-catenin, as reported previously. Both only slightly decreased beta1-integrin, the definitive receptor for fibronectin. In conclusion, the ability of PMH and S-PMH to decrease hyaluronan adhesion appears to be mediated through decreased CD44s, while the decrease in fibronectin adhesion correlates with, and may be mediated by, decreased CD44v7-10.