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 _ { m } are heavily dependent on the value of the mean projected linear size l .
For l \simeq 20 h ^ { -1 } -30 h ^ { -1 } pc , we find \Omega _ { m } \leq 0.62 , \omega \leq - 0.2 and \Omega _ { 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 _ { m } and \omega , the conventional flat \Lambda CDM model ( \omega = -1 ) with \Omega _ { m } = 0.2 and l = 22.6 h ^ { -1 } pc is the best fit for these angular size data .