Most of the presently available cancer markers are neither specific for malignancy nor allow early diagnosis. However, the recent elucidation of the molecular events occurring during tumorigenesis may provide new markers that are likely to be both specific for cancer and sensitive for early disease. The key molecules undergoing alterations during carcinogenesis are the cellular oncogenes and suppressor genes. Alterations in these genes can be detected in cells shed from malignant and premalignant lesions. Thus, mutant p53 genes have been found in urine from patients with bladder cancer, mutant ras genes in stools from patients with colorectal and pancreatic cancers, and both mutant p53 and ras genes in sputum from patients with lung cancer. These findings show that the genetic alterations in cancer can be detected in fluids or secretions that had contact with the malignant tissue. The preliminary studies, however, had small numbers of both patients and controls and used time-consuming, labor-intensive, and expensive assays. For routine applications, these assays must be simplified, automated, and tested for sensitivity, specificity, and predictive value.