Counting Items In A Hierarchical Category And Subcategories

I have two tables, one is a list of categories, the other a list of items listed in the categories. The category table is self-referencing through a ParentID column. Top-level categories have ParentID = 0.

Categories
========
ID int
ParentID int
CatName varchar(30)

Items
====
ID
CategoryID
ItemID

There is a third table that links to items through ItemID, but this is not important for this problem!

What I want to do is create a stored SQL procedure. This procedure simply pulls rows from the category table and counts the number of items that are in it. This is straighforward enough using COUNT as a "virtual column", but the difficulty is counting the items in the category but also the items in any subcategories.

An end result with two top-level categories and three subcategories in each might look like:

- Category 1 (20)
---- Subcategory 1.1 (10)
---- Subcategory 1.2 (5)
---- Subcategory 1.3 (5)
- Category 2 (14)
---- Subcategory 2.1 (3)
---- Subcategory 2.2 (4)
---- Subcategory 2.3 (7)

Hence the difficult bit is getting the total number in brackets for each category with subcategories.

What I have at the moment is:

SELECT Categories.ID, Categories.CategoryName, Categories.ParentID, (SELECT COUNT(*) FROM Items WHERE Items.CategoryID = Categories.ID) AS ItemTotal
FROM Categories

What I would like is something along the lines of (pseudocode):

SELECT Categories.ID, Categories.CategoryName, Categories.ParentID, (SELECT COUNT(*) FROM Items WHERE <Item is in category or subcategory>) AS ItemTotal
FROM Categories

I don't particularly want to use temporary tables and I definitely do not want to do any of this retrieval in my application - it needs to come straight from the database.

Thanks!