Par-4 is a widely expressed leucine zipper protein that confers sensitization to apoptosis induced by exogenous insults. Because the expression of genes that promote apoptosis may be down-regulated during tumorigenesis, we sought to examine the expression of Par-4 in human tumors. We present here evidence that Par-4 protein levels were severely decreased in human renal cell carcinoma specimens relative to normal tubular cells. Replenishment of Par-4 protein levels in renal cell carcinoma cell lines conferred sensitivity to apoptosis. Because apoptosis may serve as a defense mechanism against malignant transformation or progression, decreased expression of Par-4 may contribute to the pathophysiology of renal cell carcinoma.