Dim stDocName As String
Dim stLinkCriteria As String
stDocName = "Your2ndFormName"
stLinkCriteria = "[strID]=" & "'" & Me![MainID] & "'"
DoCmd.OpenForm stDocName, , , stLinkCriteria
Where MainID is the field on your original form holding the id'ing data
and strID is the field in the field in the second form holding (hopefully)
the same data! Probably a more elegant way to do this, but I think this'll do!

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There's ALWAYS more than one way to skin a cat!
Answers/posts based on Access 2000/2003