  Local measurements of the Hubble expansion rate are affected by structures
like galaxy clusters or voids. Here we present a fully relativistic treatment
of this effect, studying how clustering modifies the mean distance
(modulus)-redshift relation and its dispersion in a standard $\Lambda$CDM
universe. The best estimates of the local expansion rate stem from supernova
observations at small redshifts (0.01<z<0.1). It is interesting to compare
these local measurements with global fits to data from cosmic microwave
background anisotropies. In particular, we argue that cosmic variance (i.e. the
effects of the local structure) is of the same order of magnitude as the
current observational errors and must be taken into account in local
measurements of the Hubble expansion rate.
