There's more... Try the performance if this (remember the execution plan will be saved for this query in proc).

SELECT	DISTINCT a.*

FROM	(

SELECT	Case30.Case_no#,

	Case30.Last_name,

	Case30.[First Name],

	Case30.Determination,

	Case30.Sender,

	Case30.[Case Type],

	Case30.Date,

	Case30.[Qwik Due],

	Case30.[Qwik sent date],

	Case30.[Final due],

	Case30.[Final sent date],

	Case30.[on hold code],

	tblFileNames.reportid,

	tblFileNames.type,

	userviewed.viewid

FROM	tblFileNames

	Left Outer Join userviewed ON tblFileNames.reportid = userviewed.reportid

	Right Outer Join Case30 ON tblFileNames.case_id = Case30.Case_no#

WHERE 	(@param1 IS NULL OR (@param1 IS NOT NULL AND Case_no# = @param1))

AND	(@param2 IS NULL OR (@param2 IS NOT NULL AND last_name = @param2))

AND	(@param3 IS NULL OR (@param3 IS NOT NULL AND [First name] = @param3))

AND	(@param4 IS NULL OR (@param4 IS NOT NULL AND Determination = @param4))

AND	(@param5 IS NULL OR (@param5 IS NOT NULL AND sender = @param5))

AND	(@param6 IS NULL OR (@param6 IS NOT NULL AND [Case Type] = @param6))

AND	(@param7 IS NULL OR (@param7 IS NOT NULL AND Date = @param7))

AND	(@param8 IS NULL OR (@param8 IS NOT NULL AND [Qwik Due] = @param8))

AND	(@param9 IS NULL OR (@param9 IS NOT NULL AND [Qwik sent date] = @param9))

AND	(@param10 IS NULL OR (@param10 IS NOT NULL AND [Final due] = @param10))

AND	(@param11 IS NULL OR (@param11 IS NOT NULL AND [Final sent date] = @param11))

AND	(@param12 IS NULL OR (@param12 IS NOT NULL AND [on hold code] = @param12))



UNION



SELECT	DISTINCT CASEOLDR.Case_no#,

	CASEOLDR.Last_name,

	CASEOLDR.[First Name],

	CASEOLDR.Determination,

	CASEOLDR.Sender,

	CASEOLDR.[Case Type],

	CASEOLDR.Date,

	CASEOLDR.[Qwik Due],

	CASEOLDR.[Qwik sent date],

	CASEOLDR.[Final due],

	CASEOLDR.[Final sent date],

	CASEOLDR.[on hold code],

	tblFileNames.reportid,

	tblFileNames.type,

	userviewed.viewid

FROM	tblFileNames

	Left Outer Join userviewed ON tblFileNames.reportid = userviewed.reportid

	Right Outer Join CASEOLDR ON CASEOLDR.Case_no# = tblFileNames.case_id

WHERE 	(@param1 IS NULL OR (@param1 IS NOT NULL AND Case_no# = @param1))

AND	(@param2 IS NULL OR (@param2 IS NOT NULL AND last_name = @param2))

AND	(@param3 IS NULL OR (@param3 IS NOT NULL AND [First name] = @param3))

AND	(@param4 IS NULL OR (@param4 IS NOT NULL AND Determination = @param4))

AND	(@param5 IS NULL OR (@param5 IS NOT NULL AND sender = @param5))

AND	(@param6 IS NULL OR (@param6 IS NOT NULL AND [Case Type] = @param6))

AND	(@param7 IS NULL OR (@param7 IS NOT NULL AND Date = @param7))

AND	(@param8 IS NULL OR (@param8 IS NOT NULL AND [Qwik Due] = @param8))

AND	(@param9 IS NULL OR (@param9 IS NOT NULL AND [Qwik sent date] = @param9))

AND	(@param10 IS NULL OR (@param10 IS NOT NULL AND [Final due] = @param10))

AND	(@param11 IS NULL OR (@param11 IS NOT NULL AND [Final sent date] = @param11))

AND	(@param12 IS NULL OR (@param12 IS NOT NULL AND [on hold code] = @param12))

	) a