Alteration of sialyltransferase expression has been implicated in carcinogenesis. Out of sialyltransferases cloned to date, we focused on ST3Gal IV expression in human renal cell carcinoma (RCC). Levels of ST3Gal IV mRNA were examined in human RCC in comparison with non-tumor kidney. ST3Gal IV cDNA obtained by polymerase chain reaction from cDNA library of human RCC cell line ACHN was identical to STZ in nucleotide sequence. Northern blot analysis was performed for 24 non-tumor kidney and 25 primary RCC tissues, and 5 metastases. ST3Gal IV mRNA level was decreased in 16 cases of 22 primary RCC tissues compared to 21 non-tumor kidney tissues. The mRNA level was low in 4 and equivocal in one, of 5 metastases. The 6 cases that possessed almost the same levels of ST3Gal IV mRNA in primary tumor tissues as those in non-tumor kidneys showed favorable prognoses, as assessed by Kaplan-Meier curve. These results indicate that down-regulation of ST3Gal IV mRNA may be one of the factors associated with the malignant progression of human RCC.