The formal definition of "morbid obesity" is expressed in terms of body mass index (BMI), which is a ratio of height and weight. Tho with a BMI of 40 or more are considered morbidly obese. Those with BMI's over 35 who have an obesity-related medical problem(s) (co-morbidities) are also considered in the morbidly obese category. BMI's in this range generally correlate to ~ 100lbs over ideal body weight.

Morbid obesity is the level at which overweight patients develop medical problems as a result of their weight. Patients at this level of obesity are at risk of developing or worsening medical problems such as hypertension, diabetes, coronary artery disease, obstructive sleep apnea, degenerative joint disease, and certain cancers. Ultimately, these patients are at higher risk of early death from all causes.

Weight reduction to the non-morbidly obese range in morbidly obese patients has been shown to correlate with decreased severity or elimination of many of the comorbidities referenced above. There is emerging evidence that decrease in these associated medical problems is associated with longer life expectancy.

Diet and exercise regimens are still the "first line" treatment for obesity. This can be supplemented with behavioral therapy and several medicines to aid in weight loss. Though modest weight loss (~10% of total body weight) has been demonstrated with non-surgical modalities, a very high rate (~95%) of weight regain following diet termination is documented.

The National Institutes of Health have concluded that morbid obesity is a disease that has a negative impact on longevity. In a consensus statement originally published in 1991 the NIH stated that obesity surgery was an acceptable option in the morbidly obese patient who has failed non-surgical weight-control measures.