When I refer to cells through VBA usually I use the
following syntax without including the .xls extension:
Workbooks("myworkbook").Worksheets("mysheet").cell s(1,1)

Two times I have gotten an error that I was able to
resolve by adding in the .xls extension to all of my
references.
Workbooks("myworkbook.xls").Worksheets("mysheet"). cells
(1,1)

One time it was on my computer that had run that
particular macro and file many times before. Today a
coworker had a problem running another macro that I and
several other of my coworkers had no problems running.

I know that I could go into all my macros and add in
the .xls extension but I would still like to understand
what is causing this. Does anyone know why? Is it a
setting within excel or something that another file that
may be open is causing?

thanks,

Murray Williams