Validation of respiratory inductive plethysmography (LifeShirt system) (RIP LS) for tidal volume (V T), minute ventilation (VE), and respiratory frequency (f B) was performed among people with untreated obesity hypoventilation syndrome (OHS) and controls. Measures were obtained simultaneously from RIP LS and a spirometer during two tests, and compared using Bland Altman analysis. Among 13 OHS participants (162 paired measures), RIP LS-spirometer agreement was unacceptable for V T: mean difference (MD) 3mL (1%); limits of agreement (LOA) -216 to 220mL (±36%); VE MD 0.1Lmin -1 (2%); LOA -4.1 to 4.3Lmin -1 (±36%); and f B: MD 0.2brmin -1 (2%); LOA -4.6 to 5.0brmin -1 (±27%). Among 13 controls (197 paired measures), RIP LS-spirometer agreement was acceptable for f B: MD -0.1brmin -1 (-1%); LOA -1.2 to 1.1brmin -1 (±12%), but unacceptable for V T: MD 5mL (1%); LOA -160 to 169mL (±20%) and VE: MD 0.1Lmin -1 (1%); LOA -1.4 to 1.5Lmin -1 (±20%). RIP LS produces valid measures of f B among controls but not OHS patients, and is not valid for quantifying respiratory volumes among either group.