Grouping items in a ComboBox

It is possible. Use a ListCollectionView with a GroupDescription as the ItemsSource and just provide a GroupStyle to your ComboBox. See sample below:

XAML:

<Window x:Class="StackOverflow.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:StackOverflow"
        xmlns:uc="clr-namespace:StackOverflow.UserControls"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <ComboBox x:Name="comboBox">
            <ComboBox.GroupStyle>
                <GroupStyle>
                    <GroupStyle.HeaderTemplate>
                        <DataTemplate>
                            <TextBlock Text="{Binding Name}"/>
                        </DataTemplate>
                    </GroupStyle.HeaderTemplate>
                </GroupStyle>
            </ComboBox.GroupStyle>
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Name}"/>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>
    </StackPanel>
</Window>

Code-behind:

namespace StackOverflow
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {

        public MainWindow()
        {
            InitializeComponent();
            //this.comboBox.DataContext = this;

            List<Item> items = new List<Item>();
            items.Add(new Item() { Name = "Item1", Category = "A" });
            items.Add(new Item() { Name = "Item2", Category = "A" });
            items.Add(new Item() { Name = "Item3", Category = "A" });
            items.Add(new Item() { Name = "Item4", Category = "B" });
            items.Add(new Item() { Name = "Item5", Category = "B" });

            ListCollectionView lcv = new ListCollectionView(items);
            lcv.GroupDescriptions.Add(new PropertyGroupDescription("Category"));

            this.comboBox.ItemsSource = lcv;
        }


    }

    public class Item
    {
        public string Name { get; set; }
        public string Category { get; set; }
    }

}