A diagnostic method is presented that makes it possible to distinguish patients who are most likely to show a positive response to treatment with anticonvulsant medication, thereby cutting across many DSM-III-R diagnoses. Patients are evaluated to determine whether they exhibit at least 4 of 12 groups of symptoms, and the local anesthetic procaine is used, along with an electroencephalogram (EEG) to evaluate for omega band activity (30-50 Hz) of at least 50 microvolts or approximately three times baseline values in the anterior temporal leads. This method was studied in 145 patients with varying diagnoses. Eighty-three percent of patients who were symptom- and procaine-positive responded to anticonvulsants. Specific application to patients with attention- deficit hyperactivity disorder, bipolar disorder, and panic disorder is made. This method may provide a basis for identifying subpopulations of anticonvulsant-responsive patients who often are considered treatment- resistant.