1. Department of Pediatrics, Medical Centre Alkmaar, Alkmaar, The Netherlands
2. Department of Surgery, Medical Centre Alkmaar, Alkmaar, The Netherlands
3. Department of Urology, Medical Centre Alkmaar, Alkmaar, The Netherlands
4. Department of Pediatrics, Medical Centre Alkmaar, Wilhelminalaan 12, NL-1815 JD Alkmaar, The Netherlands. Fax: +31 72 5 48 21 90; E-mail: w.hack@mca.nl