It works now. I put a both commands...
Application.Volatile at the top of my custom function
and ActiveSheet.Calculate in the Worksheet_Activate event of the summary
sheet.
The Calculate without the Volatile did not work every time. It seems it works
only at first time.

MarcL wrote:
Hi Ken,
Thank you for your answer :o)
Yes, I tried with Application.Volatile but it does not work. I also tried to
put a Activesheet.Calculate in the Worksheet_Activate event, but nothing
works... except if I put Application.CalculateFull in the Worksheet_Activate
event... but it's not what I want... to recalculate all open workbooks :o(
Another alternative?

Hi Marc,
Is your SumSheets() function volatile?
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Ken Johnson