Eleven women and seven men with moderate to severe chronic hyperventilation and idiopathic seizures refractory to therapeutic serum levels of anticonvulsant medication were given diaphragmatic respiration training with percent end-tidal CO2 biofeedback. The training had a rapid correcting effect on their respiration, making it comparable to that of 18 asymptomatic control subjects. Ten of the seizure-group subjects were in the study at least 7 months and following treatment, 8 showed EEG power spectrum "normalization", restoration of cardio-respiratory synchrony (RSA), and their seizure frequency and severity were significantly reduced.