This study aims to examine the trend in the diagnosis of obesity and the use of surgical treatment in in-patient settings as well as per person and national costs associated with the surgical treatment of obesity. We conducted cross-sectional and trend analyses of in-patient obesity diagnosis and surgical treatment for obesity using data from the National Inpatient Sample (2011-2014) of adult patients in the United States aged 18 years and older. We studied the rate of in-patient obesity diagnosis among hospitalized patients, the rate of bariatric surgery among patients diagnosed with obesity in the hospital, and the costs associated with surgical treatment. Trend analyses showed a statistically significant increase in the proportion of (a) hospitalized patients diagnosed with obesity, and (b) bariatric surgery among those diagnosed with obesity and among different socio-demographic and insurance groups. Despite the increase in the national in-patient cost, the average in-patient cost per hospitalization associated with bariatric surgery decreased from 2012 to 2014. With the increase in the rate of diagnosed obesity and bariatric surgery among hospitalized patients and the decrease in the average in-patient cost, future research should address the short- and long-term cost-effectiveness of bariatric surgery on chronic diseases. © 2020 World Obesity Federation.