INTRODUCTION AND OBJECTIVE: Obesity is a highly morbid and fatal syndrome that reduces respiratory function. Obstructive sleep apnea syndrome (OSAS) is a common sleep disorder in morbid obesity. Herein, we aimed to determine how respiratory function tests changed over time after bariatric surgery and to assess non-PSG (polysomnography) tests, namely STOP-Bang questionnaire and Epworth sleepiness tests, for predicting OSAS risk. METHOD: This retrospectively conducted study enrolled 35 patients who underwent bariatric surgery. Patients were divided into three groups formed on the basis of time passed after surgery (1, 2, or 3 years). Preoperative and postoperative respiratory function test parameters assessed by spirometry, body mass index (BMI), STOP-Bang questionnaire, and Epworth sleepiness test scores were recorded. RESULTS: Twenty-four (68.6%) patients were female, 11 (31.4%) male. The mean age was 36.5±10.5 years. Postoperative weight loss of the study groups was 26% p=0.001, 23.6% p=0.002 and 25.9% p=0.005. Reductions in BMI were 32 kg/m2 p=0.001, 34.5 kg/m2 p=0.002, 35.8 kg/m2 p=0.005 respectively. Postoperative FVC (440 ml, 390 ml, 430 ml p = 0.005) and FEV1 (220 ml p = 0.005, 250 ml p = 0.004, 214 ml p = 0.005) increased in all three groups. STOP-Bang questionnaire and Epworth sleepiness scale scores significantly decreased after weight loss compared to preoperative period in all the study groups. CONCLUSION: We showed that FVC and FEV1 increased in the short and long term after weight loss by bariatric surgery; we also found that STOP-Bang questionnaire and Epworth sleepiness scale scores decreased postoperatively. These tests may be helpful to assess OSAS risk before and after surgery.