I forgot to reset Startrow

[vba]

Public Sub ProcessData()
Dim Startrow As Long
Dim Lastrow As Long
Dim i As Long

Application.ScreenUpdating = False

With ActiveSheet

Lastrow = .Cells(.Rows.Count, "A").End(xlUp).Row
Startrow = 1
For i = 1 To Lastrow + 1

If .Cells(i, "A").Value2 = "" Then

.Cells(i, "B").Formula = "=SUM(B" & Startrow & ":B" & i - 1 & ")"
Startrow = i +1
End If
Next i
End With

Application.ScreenUpdating = True
End Sub
[/vba]

BTW, are tyhe column A values really repeating or is that just illustrative?