It has been proposed that proteases secreted by cancer cells facilitate tumor invasion and metastasis by degrading the components of extracellular membranes. The lysosomal cysteine protease cathepsin L is synthesized in large amounts and secreted by many malignantly transformed cells in culture. The secreted protease is potent in degrading collagen, laminin, elastin, and other structural proteins of basement membranes. To determine whether human cancers synthesize cathepsin L, the expression of cathepsin L in approximately 100 human tumor samples was determined by quantitative RNA slot blot analysis using a specific human cathepsin L complementary DNA probe. Results of the present study suggest that cancers in general express higher levels of cathepsin L than do normal tissues. Kidney and testicular tumors expressed the highest levels of cathepsin L; non-small cell carcinomas of the lung expressed the next highest levels; and most cancers of the breast, ovary, colon, adrenal, bladder, prostate, and thyroid expressed elevated levels as well. Cathepsin L may prove useful as a diagnostic or prognostic marker of human malignancy.