Hi David,

Try

Option Explicit

Sub missing_numbers()
Dim ri As Range
Dim lmin As Long, lmax As Long, i As Long, j As Long, lrow As Long
Dim coll As New Collection

lmin = 2000000000
lmax = -2000000000
Range("B:B").ClearContents
For Each ri In Range("A:A")
If Not IsEmpty(ri) Then
If ri.Value lmax Then lmax = ri.Value
If ri.Value < lmin Then lmin = ri.Value
coll.Add 0, "X" & ri
End If
Next ri

On Error Resume Next
lrow = 1
For i = lmin To lmax
Err.Clear
j = coll("X" & i)
If Err.Number < 0 Then
Cells(lrow, 2).Formula = i
lrow = lrow + 1
End If
Next i

End Sub

This macro reads numbers in column A and writes missing numbers into
column B.

HTH,
Bernd