By growing I mean in percent used. the extents don't seem to be expiring.

When I check dba_undo_extent It has several unexpired extents. Could that be causing the undo space to become 99 percent full?

The free extents has been steadily decreasing since last November. It looks to me like there is an old uncommitted transaction hung up somewhere but I have stopped and started the instance a couple of times over the last few days and it still remains 99% full.

Is this a reason to worry or will oracle reuse the unexpired extents eventually and I shouldn't worry about it too much.