One thing I would suggest is to put that function into a module (or Class module if you have many similar procedures) and make the function's return type a Boolean.

i.e.

[vba]
PublicFunction ReadFile() As Boolean
Dim dbs As DAO.Database
Dim rst As DAO.Recordset
Dim strRecordSource AsString'Source for recordset, can be SQL, table, or saved query
Dim intFileDesc AsInteger'File descriptor for source file
Dim strSourceFile AsString'Full path of source file
Dim vIn AsString'Input buffer
Dim strField75 AsString

' Name of your file and table.
strSourceFile = "c:\subinf\ca.txt"
strRecordSource = "az"
Set dbs = CurrentDb

Set rst = dbs.OpenRecordset(strRecordSource, dbOpenDynaset)
intFileDesc = FreeFile
Open strSourceFile ForInputAs #intFileDesc
DoWhileNot EOF(intFileDesc)
Line Input #intFileDesc, vIn

' Add record in rst and fill fields:
rst.AddNew
rst.Fields("SUBSTS") = Mid(vIn, 1, 1)
rst.Fields("SUBCO#") = Mid(vIn, 2, 3)
rst.Fields("SUBEXC") = Mid(vIn, 5, 7)
rst.Fields("SUBLN#") = Mid(vIn, 12, 5)
rst.Fields("SUBRES") = Mid(vIn, 17, 2)
rst.Fields("SUBLSN") = Mid(vIn, 19, 15)
rst.Fields("SUBFRN") = Mid(vIn, 34, 15)
rst.Fields("SUBAD1") = Mid(vIn, 49, 30)
rst.Fields("SUBAD2") = Mid(vIn, 79, 30)
rst.Fields("SUBAD3") = Mid(vIn, 109, 30)
rst.Fields("SUBCTY") = Mid(vIn, 139, 20)
rst.Fields("SUBSAB") = Mid(vIn, 159, 2)
rst.Fields("SUBZCD") = Mid(vIn, 161, 10)
rst.Fields("SUBFTX") = Mid(vIn, 171, 1)
rst.Fields("SUBSTX") = Mid(vIn, 172, 2)
rst.Fields("SUBBLK") = Mid(vIn, 174, 3)
rst.Fields("SUBSTP") = Mid(vIn, 177, 2)
rst.Fields("SUBBMT") = Mid(vIn, 179, 1)
rst.Fields("SUBCRT") = Mid(vIn, 180, 1)
rst.Fields("SUBLCR") = Mid(vIn, 181, 1)
rst.Fields("SUBSNI") = Mid(vIn, 182, 3)
rst.Fields("SUBDNP") = Mid(vIn, 185, 3)
rst.Fields("SUBRCN") = Mid(vIn, 188, 3)
rst.Fields("SUBLTD") = Mid(vIn, 191, 9)
rst.Fields("SUBPSN") = Mid(vIn, 200, 3)
rst.Fields("SUBPRC") = Mid(vIn, 203, 3)
rst.Fields("SUBPDC") = Mid(vIn, 206, 3)
rst.Fields("SUBTLM") = Mid(vIn, 209, 1)
rst.Fields("SUBSCD") = Mid(vIn, 210, 3)
rst.Fields("SUBCCD") = Mid(vIn, 213, 3)
rst.Fields("SUBCDT") = Mid(vIn, 216, 9)
rst.Fields("SUBDDT") = Mid(vIn, 225, 9)
rst.Fields("SUBQBL") = Mid(vIn, 234, 3)
rst.Fields("SUBDTB") = Mid(vIn, 237, 1)
rst.Fields("SUBDLQ") = Mid(vIn, 238, 3)
rst.Fields("SUBBK#") = Mid(vIn, 241, 11)
rst.Fields("SUBTDS") = Mid(vIn, 252, 2)
rst.Fields("SUBBAC") = Mid(vIn, 254, 11)
rst.Fields("SUBHCD") = Mid(vIn, 265, 1)
rst.Fields("SUBAC#") = Mid(vIn, 266, 11)
rst.Fields("SUBLBD") = Mid(vIn, 277, 9)
rst.Fields("SUBLBM") = Mid(vIn, 286, 1)
rst.Fields("SUBLBC") = Mid(vIn, 287, 3)
rst.Fields("SUBTAD") = Mid(vIn, 290, 11)
rst.Fields("SUBSA#") = Mid(vIn, 301, 8)
rst.Fields("SUBBD#") = Mid(vIn, 309, 3)
rst.Fields("SUBBS#") = Mid(vIn, 312, 5)
rst.Fields("SUBE01") = Mid(vIn, 317, 13)
rst.Fields("SUBE02") = Mid(vIn, 330, 13)
rst.Fields("SUBE03") = Mid(vIn, 343, 13)
rst.Fields("SUBE04") = Mid(vIn, 356, 13)
rst.Fields("SUBE05") = Mid(vIn, 369, 13)
rst.Fields("SUBE06") = Mid(vIn, 382, 13)
rst.Fields("SUBE07") = Mid(vIn, 395, 13)
rst.Fields("SUBE08") = Mid(vIn, 408, 13)
rst.Fields("SUBE09") = Mid(vIn, 421, 13)
rst.Fields("SUBE10") = Mid(vIn, 434, 13)
rst.Fields("SUBSOA") = Mid(vIn, 447, 1)
rst.Fields("SUBLNG") = Mid(vIn, 448, 2)
rst.Fields("SUBNMF") = Mid(vIn, 450, 1)
rst.Update
Loop
Close #intFileDesc
rst.Close
ReadFile = True

Exit_ReadFile:
Set rst = Nothing
Set dbs = Nothing
Exit Function

Err_ReadFile:
ReadFile = False
Resume Exit_ReadFile

End Function[/vba]
Now, from the form where you call this, you only need to do do:

[vba]If ReadFile Then
MsgBox "Import was successful.", vbInformation
Else
MsgBox "Import not successful.", vbExclamation
End If[/vba]