In 17 patients with myotonic dystrophy, spirometric, flow-volume, and maximal mouth occlusion variables were obtained and compared with 8 normal subjects. Ventilatory CO2 response was measured by the estimation of the steady-state effect of a sufficiently large serial dead space. Variability of resting breathing pattern was expressed by the variation coefficients of respiratory cycle time and tidal volume. The group means of the total lung capacity (TLC), vital capacity (VC), forced expiratory volume in 1 sec (FEV)1 and forced inspiratory volume in 1 sec (FIV)1 showed a restrictive pattern. Only maximal static mouth pressure (Pi,max), measured at residual volume (RV) level, showed a significant positive correlation with both VC (p = 0.03) and FIV1 (p = 0.02), suggesting inspiratory muscle weakness as a determinant of the restriction. Although the differences were just not significant, both variation coefficients of the respiratory cycle time and tidal volume were larger in the group with a CO2 sensitivity below the lower limit of normal compared to those with a normal ventilatory response to CO2. In 3 patients, fluctuations in FRC were also present. We hypothesize that, in addition to the already documented FRC fluctuations by uncoordinated spontaneous intercostal muscle action, a defect of integration of afferent neural input and chemical drive in the medullary region may also be present in these patients.