A noninvasive measurement of pharyngeal cross-sectional area (CSA) during sleep would be advantageous for research studies. We hypothesized that CSA could be calculated from the measured pharyngeal pressure and flow using finite element analysis (FEA). The retropalatal airway was visualized using a fiberoptic scope to obtain the measured CSA (mCSA). Flow was measured using a pneumotachometer and pharyngeal pressure was measured using a pressure catheter at the palatal rim. FEA was performed as follows: using a three-dimensional image of the upper airway, a mesh of finite elements was created. Specialized software was used to allow the simultaneous calculation of velocity and area for each element using the measured pressure and flow. In the development phase, 677 simultaneous measurements of CSA, pressure and flow from one subject during NREM and REM sleep were entered into the software to determine a series of equations, based upon the continuity and momentum equations, that could calculate the CSA (cCSA). In the validation phase, the final equations were used to calculate the CSA from 1767 simultaneous measurements of pressure and flow obtained during wakefulness, NREM and REM sleep from 14 subjects. In both phases, mCSA and cCSA were compared using Bland- Altman analysis. For the development breaths, the mean difference between mCSA and cCSA was 0.0 mm² (95% CI, -0.1, 0.1 mm²). For the NREM validation breaths, the mean difference between mCSA and cCSA was 1.1 mm² (95%CI 1.3, 1.5 mm²). Pharyngeal CSA can be accurately calculated from measured pharyngeal pressure and flow using FEA.