An electronic portal imaging device (EPID) was used to detect patient setup displacement during the course of a 3-field pelvic irradiation of two groups of patients: 10 rectal and 10 prostate carcinomas. These patients were irradiated with conventional treatment techniques in routine clinical practice. A total of 469 portal images and 60 simulator films were used to determine the values of setup deviations in the X- Y- and Z-directions of a fixed coordinate system, corresponding to the medio-lateral, cranio-caudal and antero-posterior direction, respectively. The absolute displacement averaged over all setups and patients ranged between 0.4 mm and 1.4 mm with a standard deviation (S.D.) of 1.6-3.9 mm. The overall distribution along each direction could be separated into a distribution of random deviations (S.D.s ranging from 1.2 to 2.8 mm) around the mean deviation of each patient and a distribution of the means themselves: the distribution of systematic deviations (S.D.s ranging from 1.0 to 2.6 mm). Significant gradual displacement as a function of time was detected in 5 out of the 20 patients, 2 in the rectum and 3 in the prostate group. This "time trend" was found along each of the 3 directions specified. The magnitude of the time-dependent displacement throughout the course of treatment ranged between 4 and 11 mm. It can be concluded that for treatments requiring a high level of precision, portal images should be made and analyzed during the whole treatment course in order to detect and correct significant time trends.