jquery ui autocomplete: remote source problem

Im using jquery UI autocomplete.

When i use a local variable as source everything works as it should.

var json = [{ value: "1", ... etc
...
source: json

But when i return the same source from another file, it doesnt work. I find the JSON object in firebug, but when i type

source: 'path/to/filethatreturnsjson...'

It doesnt work. Did i miss something? THanks