In a previous large-scale gene expression profiling study of renal epithelial neoplasms, human beta-defensin-1 (DEFB1) was found to be significantly down-regulated in conventional clear cell (renal) carcinoma. We have now completed an expanded expression analysis of this gene. We performed immunohistochemical analysis for the DEFB1 protein in clinical specimens of both renal cell carcinoma and prostate cancer. In a subset of prostate cancers, we performed laser capture microdissection and RT-PCR to correlate mRNA levels with protein levels. Overall, 82% of prostate cancers exhibit either complete loss of protein expression or only minimal expression, whereas the adjacent benign epithelium retained expression in all cases. Similarly, 90% of renal cell carcinomas show cancer-specific loss of DEFB1 protein. In the prostate cancer subset analysis, mRNA levels correlate with protein levels. We have thus demonstrated the cancer-specific down-regulation of DEFB1 in a large sample of prostatic and renal carcinomas and validated one of the key findings of previous cancer gene profiling studies of prostatic and renal neoplasia.