Question : User.isInRole always return false

Hi, I have the following problem...
I have  Asp.net Mvc2 App (C#) on a IIS6 server (win2003). Anonymous
access is not allowed, only using windows authentication (intranet app),

my code :
System.Security.Principal.IPrincipal  User;
 User = System.Web.HttpContext.Current.User;
if (User.IsInRole("AMU\\DentalReg")) //    always return false


and i'm sure the logged user is amember from this security group

When I  change group to "domain admins" -> it runs fine

Does anyone know more about this issue?

Answer : User.isInRole always return false

By the way that is 32,767.