have you tried
copy the blank in c2 and paste in c4? jsut to see if theres any difference?

try this real quick
=countblank(c2)
should give you a 1

=countblank(c4)
should also give you a 1

if it does, then change your formula to
=if(countblank(a1)=0,"x","")

or

since c is a email address try using the if to look at the length
=if(len(a1)>3,"x","")

A    B    C    D
as    tr    cf   x
er    wr
sg    rt   yu   x
sd    ff          x
df    qw   cv   x
In the above array, columns A,B, & C are formulas. Theey represent a first
name, last name and email address, respectively. The formula for column D is
=if(C2<>"","x",""). I've replicated it down the column.

When note that line 3 works correctly, but line 5 does not. Nor does the
formula work in any following rows.

I tried the same thing, programming in VB, with similar results. This must
be a setup problem, but I can't figure it out.
I'm using Office 2007 SP1,  and Vista Home Professional.

Any suggestions would be appreciated...

Jim
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That's strange behavior indeed, but then again I'm using XL2003.  Assuming
that the first row is headers (i.e. first name, last name, email address), I
attempted to replicate your results and your formula   =IF(C2<>"","x","")
placed in  D2 worked perfectly all the way down, producing an x whenever the
email address was present and a blank if not.
Another formula  =IF(COUNTBLANK(C2)=0,"x","") also placed in D2 produced x's
as well to indicate presence of email address in column C.   Very strange.
What bothered me though was that you'd said that the data in columns A,B,
and C are *formulas* and that they *represent" first name, last name, and
email address.  If they are, in fact, formulas themselves and not actual
data, perhaps that's the root of your problem . . .
--
alan