We investigated the role of respiratory muscle incoordination during stuttering by measuring esophageal, gastric, and transdiaphragmatic pressures to obtain subglottic pressure (Psg) and indices of diaphragmatic, rib cage, and abdominal muscle contraction during speech in normal volunteers and in severe stutterers. We found in contrast to the relatively constant subglottic pressure during normal conversational speech that speech in stutterers was characterized by failure to control Psg because of contraction of the diaphragm, rib cage, and abdominal muscles singly or in various combinations. As a result, Psg varied substantially and sometimes chaotically from too high to too low, rendering normal speech impossible. During periods of fluency, Psg was much better controlled. We conclude that incoordination of the respiratory muscles is a major problem in stuttering, resulting in failure to control the pressure difference across the vocal cords. It is unclear if this is a primary abnormality or is a secondary response to a primary abnormality elsewhere.