The presence of a tumor antigen in human colonic carcinomas and their metastases was described about 25 years ago. This antigen, called carcinoembryonic antigen (CEA), is one of the first known tumor markers. Since then, many more have been described, but CEA, determined alone or in combination with others, is still one of the most used. CEA is not organ specific and abnormal values may be found in a wide range of carcinomas, especially those with gastrointestinal involvement. CEA assay should not be used for cancer diagnosis because its sensitivity in patients without cancer metastases is low. In addition, abnormal CEA values may be found in patients with benign diseases. However, the probability of malignancy increases directly with CEA concentration. Its main clinical applications are prognosis, early diagnosis of recurrence and follow-up of patients with carcinomas. In a wide range of malignancies, CEA serum levels are clearly related to tumor stage. Presurgical CEA serum levels are a well-established prognostic factor in colorectal, breast and lung cancer. Patients presenting with increased preoperative CEA serum levels have both a shorter disease-free interval and lower survival than those with normal CEA levels. In the early diagnosis of recurrence, CEA also plays an important role: in about 70-85% of patients with colorectal tumors and in 40-50% with breast cancer, CEA serial increase is the first sign of tumor recurrence. In patients with disseminated tumors, serial determinations are also a useful tool for therapy monitoring: CEA values decrease with effective treatment while stable or increasing values are observed when treatment is not effective.