Is it possible to put in "IF ...ELSE" or "Case" in WHERE CLAUSE?

Dear all...

need your help... i am now trying to create a report using SQL reporting services... when declare all the @.parameters needed in where clause, i have come across a problem. where one of the parameters that prompting user to key in...i need to put in some condition.

select..............(blah blah).....
.....(SELECT CASE WHEN
((SELECT COUNT(*)
FROM tbl_OutpatientReg OPT
WHERE OPT.PatientID = tbl_Patient.PatientID)) = 1 THEN 0 ELSE 1 END) AS PTType .....................(blah blah)......

where (CONVERT(Varchar(10), tbl_OutpatientReg.VisitDatetime, 103) BETWEEN @.FromDate AND @.ToDate) OR (@.FromDate = ' ') OR (@.ToDate = ' ')
AND (@.PatientType = CASE WHEN
(SELECT COUNT(*)
FROM tbl_OutpatientReg OPT
WHERE OPT.PatientID = tbl_Patient.PatientID) = 1 THEN 0 ELSE 1 END)

my situition is something like above, i know i have done something wrong in teh WHERE clause for the @.PatientType... can i ask how to restrict the parameters entered by user, let's say if user enter parameter "0", then the visitcount is 1, if enter "1" then the visit count refers to more than 1...

thanks in advanced ...............Your post is kind of confusing regarding requirements, but part of your problem may be due to using brackets where they are not necessary, and not using them where they might be necessary to specify logical operations.

--Your Version:
where (CONVERT(Varchar(10), tbl_OutpatientReg.VisitDatetime, 103) BETWEEN @.FromDate AND @.ToDate)
OR (@.FromDate = ' ')
OR (@.ToDate = ' ')
AND (@.PatientType = CASE
WHEN (SELECT COUNT(*)
FROM tbl_OutpatientReg OPT
WHERE OPT.PatientID = tbl_Patient.PatientID) = 1 THEN 0
ELSE 1
END)

--Unnecessary brackets removed:
where CONVERT(Varchar(10), tbl_OutpatientReg.VisitDatetime, 103) BETWEEN @.FromDate AND @.ToDate
OR @.FromDate = ' '
OR @.ToDate = ' '
AND @.PatientType = CASE
WHEN (SELECT COUNT(*)
FROM tbl_OutpatientReg OPT
WHERE OPT.PatientID = tbl_Patient.PatientID) = 1 THEN 0
ELSE 1
END

--Useful brackets added:
where (CONVERT(Varchar(10), tbl_OutpatientReg.VisitDatetime, 103) BETWEEN @.FromDate AND @.ToDate
OR @.FromDate = ' '
OR @.ToDate = ' ')
AND @.PatientType = CASE
WHEN (SELECT COUNT(*)
FROM tbl_OutpatientReg OPT
WHERE OPT.PatientID = tbl_Patient.PatientID) = 1 THEN 0
ELSE 1
END|||ya...thanks for reminding me...as there are too many parameters to pass, i also confused... ;) anyway, really appreciate ur help