If you are using treecontrol or menucontrol in ASP.NET 2.0, you can achieve this easily using the built in property.
While i was working on a custom navigation tree control, i wanted to get the depth of the current node from the root. By default ASP.NET 2.0 does not have a property to get the depth. So i wrote a neat little recursion which will calculate the depth for you.
Private Function GetDepth(ByVal current As SiteMapNode) As Integer
If(Not current.ParentNode Is Nothing) Then
Return GetDepth(current.ParentNode) + 1