IIS 8.5 - ASP.NET - user isinrole not working

Hello,

We are planning to switch from IIS 6 to IIS 8.5.  The problem is the following command in ASP.NET does not seem to work on IIS8.5:

HttpContext.Current.User.IsInRole("Role")

It is allways returning false, Windows authentication is on.  in IIS 6 there is no problem.

Is there any setting in IIS that could be wrong?

Thanks.

Fredpol