& i subtitle causes atom 1.1 to stop working

I get this error using Drupal 5.15

XML Parsing Error: not well-formed Location: http://www.denmarkonline.dk/atom/feed Line Number 4, Column 43: 
<subtitle>Væsentlige politiske nyheder & tendenser, trends</subtitle>
---------------------------------------^

This is the statusreport:
Drupal 5.15
MySQL database 5.0.32
PHP 5.2.8-0.dotdeb.1
PHP register globals none
Unicode-bibliotek PHP mbstring-udvidelse
webserver Apache/2.2.3 (Debian)