  Context. Magnetic helicity is a physical quantity of great importance in the
study of astrophysical and natural plasmas. Although a density for helicity
cannot be defined, a good proxy for it is field line helicity. The appropriate
quantity for use in solar conditions is relative field line helicity (RFLH).
Aims. This work aims to study in detail the behaviour of RFLH, for the first
time, in a solar active region (AR). Methods. The target active region is the
large, eruptive AR 11158. In order to compute RFLH and all other quantities of
interest we use a non-linear force-free reconstruction of the AR coronal
magnetic field of excelent quality. Results. We find that the photospheric
morphology of RFLH is quite different than that of the magnetic field or of the
electrical current, and this is not sensitive to the chosen gauge in the
computation of RFLH. The value of helicity experiences a large decrease, 25% of
its pre-flare value, during an X-class flare of the AR, a change that is also
depicted in the photospheric morphology of RFLH. Moreover, the area of this
change coincides with the area that encompasses the flux rope, the magnetic
structure that later erupted. Conclusions. The use of RFLH can provide
important information about the value and location of the magnetic helicity
expelled from the solar atmosphere during eruptive events.
