Set the Style property to lbOwnerDrawFixed and :

procedure TForm1.ListBox1DrawItem(Control: TWinControl;
  Index: Integer; Rect: TRect; State: TOwnerDrawState);
begin
  with ListBox1 do
    begin
    Canvas.FillRect( Rect );
    if Index in [0..1] then   // for example. First two items are drawn
bold
      Canvas.Font.Style := [fsBold];
    DrawText( Canvas.Handle, @Items[Index][1], Length( Items[Index] ),
Rect, 0 );
    end;
end;

version 2

procedure TForm1.ListBox1DrawItem(Control: TWinControl; Index: Integer;
  Rect: TRect; State: TOwnerDrawState);
begin
  if (control as Tlistbox).items[index] = 'two' then
     (Control as TListBox).Canvas.Font.Style := [fsBold]
  else
     (Control as TListBox).Canvas.Font.Style := [];
  (Control as TListBox).Canvas.TextOut(Rect.Left+2, Rect.Top+1, (Control as
TListBox).Items[Index]); {display the text }

end;