Thanks a lot, it worked without the "preceding::" part.

I'm using:
@ID = not(@ID = /dfs:myFields/dfs:dataFields/dfs:Postmortems/@event_id)

Try your xpath without the "preceding::" part. You cannot call xdXDocument
is not valid as part of preceding::

not(@ID =
xdXDocument:GetDOM("Postmortems")/dfs:myFields/dfs:dataFields/dfs:Postmortems/@event_id)

This still may not work, but I'm not positive that it won't. It is possible
that this will compare against all postmortem IDs... or it might just
compare against the first ID. If it doesn't work, you can try something
like:

not(xdXDocument:GetDOM("Postmortems")/dfs:myFields/dfs:dataFields/dfs:Postmortems@event_id[.
= current()/@ID])

I have two SharePoint list:
List A: list of events
List B: list of postmortems associated to events

Both lists have an event_id column, and every event may have an associated
postmortem.

I want a form with a drop down box that shows the list of events from "List
A" that don't have a postmortem associated in "List B", using the event_id
field.

I though the query should be something like:
not(@ID =
preceding::xdXDocument:GetDOM("Postmortems")/dfs:myFields/dfs:dataFields/dfs:Postmortems/@event_id)

But infopath doesn't allow it.

How can I filter something like that?