Context : A new reduction of the astrometric data as produced by the Hipparcos mission has been published , claiming that the accuracies for nearly all stars brighter than magnitude \mathrm { Hp } = 8 are improved , by up to a factor 4 , compared to the original catalogue .
As correlations between the underlying abscissa residuals have also been reduced by more than an order of magnitude to an insignificant level , our ability to determine reliable parallaxes and proper motions for open clusters should be improved .

Aims : The new Hipparcos astrometric catalogue is used to derive mean parallax and proper motion estimates for 20 open clusters .
The HR-diagrams of the nearest clusters are compared and combined to provide future input to sets of observational isochrones .

Methods : Three different methods are applied , according to the proximity of the cluster , to compensate , where needed , for projection effects , spread in position along the line of sight , and the internal velocity dispersion of the cluster stars .

Results : The new parallaxes have accuracies between 2 and 2.5 times higher than what had been derived from the original Hipparcos catalogue .
At least two to three groups of clusters , mostly of similar ages , are observed to each occupy their own specific space in the HR diagram .
A significant discrepancy in distance moduli from those obtained with isochrone-based main-sequence fitting remains , in particular for one of these groups , containing the Pleiades , NGC 2516 , and Blanco 1 .
The difference in absolute magnitudes between this group and one containing the Hyades and Praesepe clusters appears to be correlated with systematic differences in the Strömgren \Delta c _ { 0 } index between those groups .
The same dependency has been known to exist for a long time , and is again confirmed by the Hipparcos data , in variations in absolute magnitudes for field stars of the same effective temperature .

Conclusions : The positions of the cluster HR diagrams are consistent within different groups of clusters shown for example by the near-perfect alignment of the sequences for the Hyades and Praesepe , for Coma Ber and UMa , and for the Pleiades , NGC 2516 , and Blanco 1 .
The groups are mutually consistent when systematic differences in \Delta c _ { 0 } are taken into account , where the effect of these differences on the absolute magnitudes has been calibrated using field-star observations .