To evaluate the role of ventilatory muscle endurance training (VMET) in the rehabilitation of outpatients with chronic obstructive pulmonary disease, we carried out a prospective random allocation trial of VMET versus IPPB. Data were obtained from 15 men allocated to VMET and from 17 men assigned to IPPB. The mean age of our experimental cohort was 61 +/- SEM 1 yr, and the FEV1 was 1.2 +/- 0.1 L. Prior to and after 6 wk of daily therapy, the following data were obtained on each subject: (1) vital statistics, (2) standard pulmonary function tests, (3) activities of daily living (ADL), (4) maximal sustainable ventilatory capacity (MSVC), (5) psychologic status (PS), and (6) exercise tolerance (ET). Prior to therapy, the VMET and IPPB groups showed no significant differences with respect to these parameters. After therapy, VMET subjects exhibited a greater increase (p less than 0.05) in MSVC than did IPPB subjects. However, VMET and IPPB groups did not differ with respect to improvements noted in ADL, PS, and ET. These results from our controlled study raise the possibility that some aspect of the experimental protocol, other than VMET, accounted for the improvements noted in ADL, PS, and ET.