BACKGROUND To determine the proportion of patients with obstructive sleep apnea (OSA) who have leg edema, and to identify differences between edematous and non-edematous OSA patients. METHODS Retrospective, cross-sectional study of 378 patients with OSA (apnea/hypopnea index [AHI] >or=15) who had neither heart failure nor chronic lung disease. RESULTS Thirty-five percent (133/378) of the subjects with OSA had bilateral leg edema. Eighty-one percent (108/133) of the edematous subjects had mild pitting that was 1+. Compared to the non-edematous OSA subjects, the edematous subjects were older (age=51+/-13 versus 45+/-13 years, p=0.001), more obese (body mass index=39+/-9 versus 33+/-8kg/m(2), p=0.001), had more severe OSA (AHI=46+/-71 versus 27+/-29, p=0.004), spent a greater proportion of sleep time with an oxygen saturation <90% (20+/-26 versus 11+/-18%, p=0.001), and were more likely to have diabetes mellitus (11% versus 3%, p=0.001) and hypertension (32% versus 10%, p=0.001). Age, obesity, hypertension and diabetes mellitus correlated significantly with edema status. After adjusting for these confounding variables, the AHI means remained different between the edema and non-edema groups (41+/-5 versus 28+/-3, p=0.04). CONCLUSIONS Approximately one-third of OSA patients have edema. Edematous OSA patients are older, more obese, more likely to have diabetes mellitus and hypertension, and have more severe OSA than OSA patients who lack edema.