How to sum cell accros range of sheets

[If...Then...]

Hope someone can help.

In Excell 2003: I need to sum up values in the same cell accross a range of sheets but only if a condition (only one condition) is met.

Let me explain: i have a range of sheets that i bracketed on both ends with a blank sheet.

so the arrangement is summary sheet, blankfirst sheet, a number of other sheets, blanklast sheet. If i wanted to sum up let say cell D20, on the summary sheet i would say =sum(blankfirst:blanklast!d20) and get a sum of values in this cell accross all sheets. easy enough.

what i need to do is to say sum cell D20 but pick up values only on those sheets where value of cell A5 equals the value of cell A5 on summary sheet.

i will make sure that the values of cell A5 are identical on all sheets that have it. i can make it either numeric or text with only numbers in it. All rows/columns accross all sheets will remain in place and there will be no inserts of rows/columns in the future.

what function would allow me to do this? or is there such a function? or a combination of functions? I can not use macros as this needs to display the summary at all times.

any help would be greatly appreciated. Thank you.