Results of real-time monitoring of prostate intrafractional motion during external beam radiation therapy are presented based on the observation using the Calypso 4D localization system with implanted electron-magnetic transponders in the prostate gland. Prostate displacement during treatment was sampled at a frequency of 10Hz and the data for 105 patients was analyzed. The fractional time that the prostate was displaced by more than 3, 5, 7 and 10mm was calculated for each patient. The statistical results of the prostate displacement in each direction and possible reduction from employing intrafractional intervention are discussed. About 29% of the treatment fractions needed intervention at least once if the displacement threshold was 3mm off the base line in all the three directions and the percentage was reduced to 5% after the threshold increased to 5mm. The percentage time of prostate off the base line by 3mm and 5mm in any direction is 13.4% and 1.8%, respectively, for the 105 patients investigated, and 41% and 15% respectively for 7 patients who exhibited the largest prostate motion, if no intervention was performed during the treatment. The mean and standard deviation of the prostate displacement were less than 1mm in any direction for all the patients. In contrast, the mean values for the 7 patients were significantly higher, 1.4mm in S-I direction and 2.3mm in A-P direction. Comparing with other geometrical uncertainties, the prostate intrafractional motion is not a major contributor for the general patient population and prostate motion tracking during the treatment plays a small role in the treatment margin reduction. However, a small fraction of patients who have large intrafractional motion can benefit significantly from real-time motion tracking and threshold-based intervention based on our current margin setting.