  Weak gravitational lensing by large-scale structure affects the determination
of the cosmological deceleration parameter $q_0$. We find that the lensing
induced dispersions on truly standard candles are $0.04$ and $0.02$ mag at
redshift $z=1$ and $z=0.5$, respectively, in a COBE-normalized cold dark matter
universe with $\Omega_0=0.40$, $\Lambda_0=0.6$, $H=65$km/s/Mpc and
$\sigma_8=0.79$. It is shown that one would observe $q_0=-0.44^{+0.17}_{-0.05}$
and $q_0=-0.45^{+0.10}_{-0.03}$ (the errorbars are $2\sigma$ limits) with
standard candles with zero intrinsic dispersion at redshift $z=1$ and $z=0.5$,
respectively, compared to the truth of $q_0=-0.40$ in this case, i.e., a 10\%
error in $q_0$ will be made. A standard COBE normalized $\Omega_0=1$ CDM model
would produce three times as much variance and a mixed (hot and cold) dark
matter model would lead to an intermediate result. One unique signature of this
dispersion effect is its non Gaussianity. Although the lensing induced
dispersion at lower redshift is still significantly smaller than the currently
best observed (total) dispersion of $0.12$ mag in a sample of type Ia
supernovae, selected with the multicolor light curve shape method, it becomes
significant at higher redshift. We show that there is an optimal redshift, in
the range $z\sim 0.5-2.0$ depending on the amplitude of the intrinsic
dispersion of the standard candles, at which $q_0$ can be most accurately
determined.
