How to set a validation error via ajax? I am using the formapi to create a form. On one of my fields I have an #ajax call. When the field is changed I call the function. I recreate the form and then replace the whole form on the page. This works fine.

When creating the form I have some logic that sets the form state to see if a field has a value. If it has a value then I show some more fields. If the value is not set then I don't show the fields. I am showing the fields by setting the #access on the field.

I need to add some validation on the form when the ajax call is triggered. I have tried adding the validation using form_set_error() in the function that creates the form and in the ajax callback but the error is not displayed on the screen.

Am I going about this the wrong way completely? How can I get the form error to display?
If the name of your form is modulename_admin_form() then you can add a function modulename_admin_form_validate(&amp;$form, &amp;$form_state).

In that validate function you can form_set_error() from in that validate function.