In order to evaluate the correlation between clinical progress and CEA levels in gastroenteric tumours, particularly during chemotherapeutic treatment, CEA assay was performed on 330 patients (126 with gastric neoplasms, 204 with large bowel carcinoma). 175 out of these had a pre-operative assay. Moreover CEA test positivity (CEA larger than or equal to 5 ng/ml according to the technique employed by us) is higher in colon neoplasms compared with gastric neoplasms and in adenocarcinomas in comparison with undifferentiated forms; besides it depends on the pathologic stage. In colon tumours CEA test showed a higher positivity for left than for right forms (66.6% versus 38%). 240 patients were followed up with repeated CEA assays: the following observations were made: a - After radical surgery 72% of the patients shows normalized CEA values. b - In 198 patients who underwent radical surgery, not requiring chemotherapy, there was a close correlation between CEA levels and clinical evolution in 98% of the cases. c - 60 out of 68 patients (88%) submitted to chemotherapy for advanced neoplasms show a close correlation between CEA response to the chemotherapy and clinical response (p less than 0.001). These investigations stress, above all, the importance of CEA test to monitor the treatments performed (surgical and chemotherapeutic) and to adopt, eventually, more effective chemotherapeutic modalities.