  About 10$\%$ of the massive main sequence stars have recently been found to
host a strong, large scale magnetic field. Both, the origin and the
evolutionary consequences of these fields are largely unknown. We argue that
these fields may be sufficiently strong in the deep interior of the stars to
suppress convection near the outer edge of their convective core. We performed
parametrised stellar evolution calculations and assumed a reduced size of the
convective core for stars in the mass range 16 M$_{\odot}$ to 28 M$_{\odot}$
from the zero age main sequence until core carbon depletion. We find that such
models avoid the coolest part of the main sequence band, which is usually
filled by evolutionary models that include convective core overshooting.
Furthermore, our `magnetic' models populate the blue supergiant region during
core helium burning, i.e., the post-main sequence gap left by ordinary single
star models, and some of them end their life in a position near that of the
progenitor of Supernova 1987A in the HR diagram. Further effects include a
strongly reduced luminosity during the red supergiant stage, and downward shift
of the limiting initial mass for white dwarf and neutron star formation.
