The aim of this study is to evaluate if there is an association between body weight and obstructive sleep apnea even in patients who are not obese. From our hospital database, we identified 139 patients who underwent an overnight polysomnography. The inclusion criteria were patients (i) who visited our hospital with a chief complaint of snoring or apnea that was witnessed by a sleep partner, and (ii) with a body mass index <25 kg/m2. We compared the clinical variables, polysomnographic variables, and Epworth Sleepiness Scale scores in subjects with and without obstructive sleep apnea. Additionally, correlation analysis was performed between apnea-hypopnea index and age, body weight, and neck circumference. Of the 139 patients, 62 patients met the inclusion criteria for this study. Forty-two of the 62 patients were diagnosed with obstructive sleep apnea. Patients with obstructive sleep apnea were older, had higher body weight, larger neck circumference, and greater apnea-hypopnea index, longer time of pulse oxygen saturation <90%, and lower minimum pulse oxygen saturation compared to those without obstructive sleep apnea. In addition, apnea-hypopnea index was found to be positively correlated with age, body weight, and neck circumference. Multiple logistic regression analysis showed that old age, high body weight, and large neck circumference were independently associated with obstructive sleep apnea. We demonstrated that there was a significant association between body weight and obstructive sleep apnea in non-obese patients, and further prospective study is needed to evaluate the beneficial effect of weight loss on obstructive sleep apnea in non-obese patients.