PURPOSE To investigate the effect of post-implant edema on the CT-based calculation of the total dose delivered by an I-125 prostate implant. MATERIALS AND METHODS CT scans of a transperineal I-125 prostate implant were obtained 1 and 39-days post-implant. Changes in the prostate dimensions were determined from changes in the spatial distribution of the I-125 seeds. The total dose delivered to the target volume was computed from each CT scan, and the results compared. RESULTS The volume of the prostate decreased by approximately 17% during the 38-day interval between the first and second CT scans. As a result, the radiation dose computed from the second CT scan was 13% higher. CONCLUSION Post-implant edema can cause a significant underestimation of the radiation dose delivered by an I-125 prostate implant. Similar analysis should be carried out among a larger cohort of patients to confirm or refute these observations.