populate combobox

I am currently populating a combobox as follows:

private void PopulateHealthLevels()
{
List<clsStudentDetails> healthlevels = new List<clsStudentDetails>();
healthlevels = clsStudentAccess.GetHealthLevels();
foreach (clsStudentDetails details in healthlevels)
{
cboHealth.Items.Add(details.HealthLevelName);
}
}

Question:
This is what I have in xaml:
Name="cboHealth" SelectedIndex="{Binding Path=HealthLevelID}"

The selectedindex does not seem to be working correctly.
No matter which student I choose, the item shown is always the same.
Note that there are items populated in th ecombobox.
Thanks

Use the SelectedItem property instead, and have it bound to an instance of an item in the list.