['In patients with Duchenne muscular dystrophy (DMD), implementation of mechanical ventilation depends on sleep investigation and measurement of CO2 tension. The objective of this cross-sectional study was to determine which noninvasive lung function parameter best predicts nocturnal hypercapnia and diurnal hypercapnia in these patients.', 'According to transcutaneous CO2 (TcCO2) measurement, 114 DMD patients were classified into three groups: nocturnal hypercapnia (n = 38) [group N], diurnal hypercapnia (n = 39), despite nocturnal ventilation (group D), and 24-h normocapnia and spontaneous breathing (n = 37) [group S] as control. TcCO2 tension and lung function variables included vital capacity (VC) and maximal inspiratory pressure (MIP), and breathing pattern variables included tidal volume (Vt) and respiratory rate (RR), measured at the time of group inclusion. The rapid and shallow breathing index (RSBI [RR/Vt]) and Vt/VC ratio were calculated. Areas under the curve from the receiver operating characteristic (ROC) were calculated for those parameters.', 'Compared to group S, lung function was significantly worse in group N and group D. VC, RR, and RSBI distinguished group S from group N by ROC comparison. Cut-off values of VC < or = 680 mL (ROC, 0.968), MIP < or = 22 cm H2O (ROC, 0.928), and Vt/VC > 0.33 (ROC, 0.923) accurately discriminated group D from group N, but RSBI, RR, and Vt did not.'] Does lung function accurately predict hypercapnia in patients with Duchenne muscular dystrophy?