Thanks to Steve Adams and Domingo Diaz Saenz for putting me on the right track. I was caught out by some very long (not completed) lock waits. As this earlier entry says: //www.freelists.org/archives/oracle-l/10-2005/msg01140.html > If a session is waiting for an enqueue request, the TIME_WAITED_MICRO > in v$system_event is incremented for every three seconds (when an > enqueue timeout occurs). However, v$enqueue_stat is only incremented > after the session has acquired or cancelled the enqueue resource > request. Adding times from gv$enqueue_stat + gv$session_wait should more or less equate to gv$system_event, as long as you handle time units and correct for 1024 milliseconds in a second where necessary - see Jonathan Lewis's FAQ on the subject: http://www.jlcomp.demon.co.uk/faq/time_unit.html. Thanks and regards Nigel