Purpose: The aim of this study was to verify the first IMRT prostate plans made in our clinic with a newly commissioned treatment planning system (TPS, Pinnacle 7.4f) using an amorphous siliconelectronic portal imaging device (a‐Si EPID, Elekta iViewGT), both pre‐treatment and in vivo. Method and Materials: For pre‐treatment verification, the plans of 8 patients were re‐calculated on a polystyrene slab phantom. An in‐house developed back‐projection algorithm was used to estimate the dose distribution at the phantom/patient isocentric mid‐plane (perpendicular to the beam‐axis) with the EPID. Each plan was also validated at the isocentre with ionization chamber measurements. Separate fields were measured with film and EPID, with gantry angle =0°. The in vivo mid‐plane dose was estimated with the EPID for the first 3 fractions and weekly thereafter. γ‐evaluation was used to assess 2D dose distributions with criteria of 3% dose difference (of maximum dose) and 3mm distance‐to‐agreement. The evaluated area included all points within the 20% isodose line of each EPID field. Anatomy changes for in vivo measurements were assessed using cone‐beam CT acquired prior to each verified fraction. Results: For pre‐treatment verification, the dose distributions of EPID vs. plan and EPID vs. film agreed within 3% or 3mm for 99.2% and 100% of points, respectively. The average ratio of the measured and planned isocentre dose was 0.987 ±0.003(SD) for ionization chamber and 0.997 ±0.009(SD) for EPID. For the in vivoyfields, 96.7% of dose points were in agreement. Examples of discrepancies were due to variation in gas pockets during treatment and problems calculating the dose distribution in a small area of overlapping segments (1cm2). Conclusion: These results show that an a‐Si EPID can be used to accurately verify IMRT prostate treatments in the mid‐plane of the phantom or patient, both pre‐treatment and in vivo.