New microgravity data from the active crater of Po¿s volcano, Costa Rica, collected in 2002--2004 extends the existing dataset to provide a unique 20-year time series. These data show that gravity has decreased monotonically in the north and east of the crater over the last 5 years, whilst it has increased to the west and remained approximately constant in the south. These changes are interpreted in terms of convective recharge within dendritic intrusions beneath the crater, with overall down-welling in the north and up-welling in the west. The data reveal a 5--10 year periodicity in sub-crater mass movement, but overall, the upper part of the conduit system appears to have maintained a state of mass equilibrium.