Exposure to potential carcinogens is an etiologic factor for renal cell carcinoma (RCC) and transitional cell carcinoma (TCC) of the urinary bladder. Cytosolic glutathione S-transferases (GSTs) are a superfamily of enzymes that protect normal cells by catalyzing conjugation reactions of electrophilic compounds, including carcinogens, to glutathione. Some GST enzymes possess antioxidant activity against hydroperoxides. The most well characterized classes have been named alpha (GSTA), mu (GSTM), pi (GSTP) and theta (GSTT); each of these classes contains several different isoenzymes. Several types of allelic variation have been identified within classes, with GSTM1-null, GSTT1-null and GSTP1-Ile105/Ile105 conferring impaired catalytic activity. The effects of GSTM1 and GSTT1 polymorphism on susceptibility to RCC depend on exposure to specific chemicals. Individuals with the GSTM1-null genotype carry a higher risk for TCC. The roles of GSTT1 polymorphism in TCC and GSTP1 polymorphisms in both cancers are still controversial. During kidney cancerization, expression of GSTA isoenzymes tends to decrease, which promotes the pro-oxidant environment necessary for RCC growth. In the malignant phenotype of TCC of the bladder, upregulation of various GST classes occurs. Upregulation of GSTT1 and GSTP1 might have important consequences for TCC growth by providing a reduced cellular environment and inhibition of apoptotic pathways.