I'm trying to design a table that has, among other fields, a field for define the kind of complaint my company receive from customers, and another field contains the root causes of these complaints. The root causes are more than 20, and they are especially associated with an especific kind of complaint, so I need that when I select a kind of complaint in a combo box, in the combo box related to the root causes appear only the root causes associated with the kind of complaint selected, to select one of them.

Do you plan to have a separate table to show the association of these complaints and causes? On the form you could have 2 combo boxes. The first one the customers can input the type of complaint. The second combo box will show the root causes. You can restrict what is listed by what is chosen from the first combo box. On the row source for the second combo box, build a query so that it only shows the records based on what is selected from the first combo box. (example: field name of complaint = [forms]![form name]![combo1])