The Medical Outcomes Study (MOS) inventory is widely used to assess quality of life in chronically ill patients. Although response bias was addressed during its initial validation, we are unaware ofsubsequent studies examining how personality characteristics influence responses on this instrument. We examined the impact of response bias on MOS data reported by 44 obstructive sleep apnea (OSA) outpatients who completed the MOS and Marlowe-Crowne Social Desirability Scale (MC). Social desirability implies responding in a way that the participant believes will be viewed favorably; MC is often used to statistically controlfor this form of response bias on self-report measures. The MOS yielded 2 summary scales (Mental and Physical Health) and 8 dimensions (based on Short Form-36 [SF-36]). Data were analyzed using multiple regression analyses. Results show that there is a significant relation between MC and the Mental and Physical Health factors (R2 = .194, F = 5.069, p = .011) and the eight SF-36 dimensions (R2 = .359, F= 2.448, p = .032). Post hoc analyses did not reveal that any one independent variable was a superior predictor of the MC. Participants scoring high on the MC reported 31% greater health than participants scoring low on the MC. Findings suggest that response bias has a significant influence on MOS data from OSA patients, with the greatest impact on mental health indexes. Controlling for response bias on the MOS and other self-report measures is important in both research and clinical situations with OSA and potentially other chronically ill patients.