  We investigate observational constraints on the cosmic equation of state from
measurements of angular size for a large sample of milliarcsecond compact radio
sources. The results are based on a flat Friedmann-Robertson-Walker (FRW) type
models driven by non-relativistic matter plus a smooth dark energy component
parametrized by its equation of state $p_x = \omega \rho_x$ ($-1 \leq \omega <
0$). The allowed intervals for $\omega$ and $\Omega_{\rm{m}}$ are heavily
dependent on the value of the mean projected linear size $l$. For $l \simeq
20h^{-1} - 30h^{-1}$ pc, we find $\Omega_{\rm{m}} \leq 0.62$, $\omega \leq
-0.2$ and $\Omega_{\rm{m}} \leq 0.17$, $\omega \leq -0.65$ (68% c.l.),
respectively. As a general result, this analysis shows that if one minimizes
$\chi^{2}$ for the parameters $l$, $\Omega_{\rm{m}}$ and $\omega$, the
conventional flat $\Lambda$CDM model ($\omega = -1$) with $\Omega_{\rm{m}} =
0.2$ and $l = 22.6 h^{-1}$pc is the best fit for these angular size data.
