You do it with an IF field:
{Mergefield Address1}{IF {Mergefield Address2} "" "{Mergefield
Address2}"}{Mergefield Address3}Note: in the example above there is a line
feed (Shift+Enter) following the third speech mark.

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Greg Maxey/Word MVP
A Peer in Peer to Peer Support

SandieBch wrote:
How can you omit blank lines in a merged document (such as a mailing
label) where the field has no data in the data source document?