uses
  IDFtpList;

var
  l: TIdFtpListItems;
  i: integer;
  sl: TStringList;
begin
  sl := TStringList.Create; // create raw data container
  FTPClient.Connect(true, -1); // login
  FTPClient.List(sl, '*', true); // get all entries with details
  l := FTPClient.DirectoryListing; // get ref of parsed entries
  for i := l.Count - 1 downto 0 do // start on safe side
    if l[i].ItemType = ditDirectory // check for sub directories
      then sl[i] := l[i].FileName // keep dir entries
      else sl.Delete(i); // drop other entries
  FTPClient.Disconnect; // logout
  ListBox.Items.Assign(sl); // show directory names
  sl.Free; // drop raw data container
end;