Purpose: This work uses Monte Carlo simulations to model an unexpected dosimetry effect of electron contamination in Longitudinal Field MRI���Linac systems (LF���MRIGRT). These systems are under construction due to their dosimetric advantages over transverse���field systems. However this work has yielded a potential major dosimetric problem and aims to investigate its scope. Methods: Geant4 Monte Carlo simulations of various generic longitudinal���field MRI���Linac systems are performed. The fringe���field of the MRI system is included as it collects and propagates electron contamination from the Linac towards the patient. Field size, isocentre distance, and fringe��� field style are all varied and examined. The Linac modeled is a Varian 2100c 6MV beam, and dose is scored in a 30��30��20cm3 phantom. Results: The longitudinal component of the MRI���system fringe field acts to trap electron contamination arising from the Linac collimation system. This becomes focused and cumulates as it travels towards the patient level. As a result a massive concentration of electron contamination is present near CAX at the patient level, leading to entry skin dose hot spots of > 500% of dose at dmax (15 mm). Even if the Linac collimation system is fully magnetically shielded there is resultant lack of lateral spread (or dilution) of contamination by the main MRI���field region. This still leads to skin dose hot spots approaching Dmax. Conclusion: Monte Carlo modeling predicts that electron contamination from the Linac in LF���MRIGRT may be trapped by the MRI system and cause considerable patient entry skin dose hot spots near CAX, of the order of > 500% of Dmax. It is highly recommended that accurate modeling of the MRI fringe���field and MRI���Linac system be performed to ascertain this risk in potential TF���MRI���Linac systems. �� 2011, American Association of Physicists in Medicine. All rights reserved.