Counting the number of distinct rows returned

How can I return the number of records (ie a count(user_id)) when using a
statement like this:

select distinct user_id from sales_by_region;

I have tried things like:

select count(distinct user_id) from sales_by_region (parse error)
select distinct count(user_id) from sales_by_region (useless because count
is performed before the distinct)

HELP!

Thanks,

Drew