In a prospective study of 928 patients with gastrointestinal cancer registered for primary resection, the postoperative carcinoembryonic antigen (CEA) time courses were analysed in connection with disease recurrence. Only patients with established diagnosis of disease recurrence and complete follow-up to death entered the evaluation of prognostic criteria of the CEA time course. In this group the CEA time courses of 103/201 patients with recurrent disease exhibited an exponential increase of the serum CEA concentration, i.e. a linear relationship of log CEA and time, which allowed the calculation of the CEA doubling time. All 103 patients developed metastatic spread and generally exhibited CEA doubling times ranging between 10 and 158 days, in patients developing peritoneal carcinosis up to 343 days. The individual CEA doubling times of patients with recurrent disease who received no treatment (n = 71) correlated well with the times of individual survival after the initial CEA increase of the log CEA phase (rs = 0.812; P less than 0.001) thus confirming the results of a previous retrospective study. When the survival time is expressed in multiples of the individual CEA doubling time (IDT), no patient survived longer than 10.8 IDT. The median value of survival was 5.4 IDT. Patients with metastatic spread who underwent various treatments of recurrent disease (n = 32) survived distinctly longer showing survival times up to 32.6 IDT. This could be confirmed by comparing the observed survival after the initial CEA increase of treated and untreated patients (life table method) exhibiting highly significant differences (P less than 0.001).