Hyaluronic acid (HA) levels are elevated in bladder cancer tissues and regulate tumor growth and progression. Urinary HA levels measured by the HA test are an accurate marker for bladder cancer. In cells, HA is synthesized by one of the 3 HA-synthase(s) i.e., HAS1, HAS2 and HAS3. In this study, we examined HAS1 expression in bladder cancer cells and tissues. Real-time RT-PCR and northern blot analyses showed that HAS1 transcript levels are elevated 5- to 10-fold in bladder cancer tissues, when compared with normal tissues (p < 0.001). Among the 3 HAS1 splice variants, only HAS1-va was expressed in bladder tissues, but the expression was significantly lower than the wild type HAS1 transcript. Increased HAS1 expression in bladder tumor tissues correlated with increased tissue HA levels (p < 0.001). Size of the large HA species (2.0 × 106 D) present in bladder tissues was consistent with the size of the HA polymer synthesized by HAS1. The amount of HA produced by bladder cancer cell lines correlated with the expression of HAS1 protein. Immunohistochemical analyses of bladder tumor tissues showed that HAS1 and HA expression had 79–88% sensitivity and 83.3–100% specificity. Both HAS1 and HA expression in bladder cancer tissues correlated with a positive HA urine test (p < 0.001). HAS1 expression correlated with tumor recurrence, prior treatment (p < 0.05) and possibly disease progression (p = 0.058). Therefore, elevated HAS1 expression in bladder tumor tissues contributes to a positive HA urine test and may have some prognostic potential. © 2007 Wiley-Liss, Inc.