Twenty chronic hyperventilation syndrome patients were assigned to biofeedback training to raise end-tidal PCO2 level or received a breathing exercise schedule for home practice. Both methods offered equal opportunity for improving end-tidal PCO2 significantly. Alleviation of complaints, however, was only noticeable in the biofeedback group. This may be due to the fact that improvement in the severity of the hyperventilation syndrome was not only significantly related to increase in end-tidal PCO2 but also to initial end-tidal PCO2 level and state of the ventilatory controlling system. A follow-up evaluation three months after the end of the therapy programs showed an unchanged end-tidal PCO2 level in both groups as compared with post-treatment evaluation, but a significant relapse in complaints in the biofeedback group. Implications of these findings are discussed in the final section of this paper.