Try:
edtLgMb.Lines.Count = 0
For more information, carefully review the description of the Lines property in the Help file. It should contain a link to the class in question.
Remember: Lines is tStrings descendent, which means that it's really an array of string pointers. As such, to access the specific lines in a memo, the index is zero based, e.g. Line 1 is Lines[ 0 ].
Also, keep in mind that this only tells you if there aren't any lines; if the memo contains a blank line, you'll get a 1 from from this.
Hope this helps...
-- Lance