Purpose: To investigate the dosimetric effect of prostate rotations and limited on-line corrections by couch rotations (≤3°) for prostate, seminal vesicles and organs at risk. Methods: For 5 patients IMRT plans were made, treating the prostate plus base of the seminal vesicles. Realistic and idealised dose distributions were considered, the latter demonstrating extreme effects for rotations and their corrections. Translation errors were assumed to be corrected on-line. For each patient 20 treatments with different rotation errors were simulated: 20 systematic errors were generated and 20 times 35 random deviations were superimposed to simulate day-to-day variations. Using a research module of PLATO-RTS treatments with rotation errors, with and without on-line corrections, were simulated. Results: The largest dosimetric effect of rotation errors and corrections was found for the seminal vesicles with idealised dose distribution: coverage improved from 92.6% (range 89.9-96.0%) to 95.9% (94.7-98.1%). The gain for the idealised prostate was less: 95.9% (94.4-97.0%) to 97.5% (95.5-98.4%). For the femoral heads the dose increase could be as large as 12.2% (6.2-19.3%). Conclusions: On-line correction of rotations can improve target coverage slightly. For organs at risk at a large distance from the isocentre the result can be a significant increase in dose. © 2008 Elsevier Ireland Ltd. All rights reserved.