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.