So you have a Form with a combobox (bound to a primary key) used to filter the records shown in a sub-form (filtered on a foreign key which is the same as your primary key on the combobox on the main form)?? If so, the combobox which is in the main form will always show all the available records.
I dont have a subform. The combo box is used to find a particular record in the form. I created it using the combo box wizard and selected the option "Find a record in my form based on the value I selected in the combo box". Then I created command buttons: A,B,C..... When I click button "A", the form will be filtered to all Last Names starting with "A" only and so on... I also want the combo box to show also all Last Names starting with "A". But when i click the combobox, it still shows all records with no filter.
If you open the properties for your combobox and look in the data tab under "Row Source". Click the button at the right, the one with three dots (...). You will open up a query design window. This is the query that is used to populate your combobox. You will need to add criteria to it to filter it the same way your form is being filters.
e.g. Left([LastName],1) = "T" or Left([LastName],1) = me.txtFilterOn '(where txtFilterOn is used to filter your form)