Short answer
Class 3 obesity — also referred to as severe obesity or morbid obesity — is the highest tier on the obesity scale. In adults, it is defined as a BMI of 40 or higher, and in children as an iso-BMI of 40 or higher. It is a chronic disease associated with a high risk of comorbidities such as type 2 diabetes, cardiovascular disease, sleep apnoea, and fatty liver disease. Treatment is delivered within specialist care and may include pharmacotherapy and bariatric surgery in addition to behavioural treatment.
What is class 3 obesity?
Obesity is divided into three classes based on BMI (adults) or iso-BMI (children):
| Class | BMI / iso-BMI | Description |
|---|---|---|
| Class 1 | 30–34.9 | Obesity |
| Class 2 | 35–39.9 | Severe obesity |
| Class 3 | 40+ | Extreme/severe obesity (also referred to as morbid obesity) |
Class 3 obesity is the most severe tier and carries a significantly increased risk of comorbidities.
In children
In children, iso-BMI is used — an age- and sex-adjusted BMI according to Cole 2000. An iso-BMI of 40 in children corresponds to the same relative level as a BMI of 40 in adults. For class 3 obesity in children, treatment within specialist care (BUMM [paediatric outpatient clinic], paediatric clinics, or specialised obesity units) is necessary. See obesity in children.
What are the associated risks?
With class 3 obesity, the risk is substantially elevated for, among others:
- Type 2 diabetes and insulin resistance
- Hypertension and cardiovascular disease
- Sleep apnoea
- Fatty liver disease (NAFLD)
- Joint problems
- Mental ill-health
- Certain types of cancer (in adulthood)
Life expectancy is shortened compared with normal weight if class 3 obesity remains untreated.
How is class 3 obesity treated?
Treatment is multidimensional and takes place over an extended period:
- Structured family-based treatment / behavioural treatment — always the foundation, even when other therapies are added.
- Pharmacotherapy — GLP-1 analogues (liraglutide, semaglutide) can be used in adolescents and adults; see Ozempic for children.
- Bariatric surgery — gastric bypass or sleeve gastrectomy may be considered in adults and in selected cases in older adolescents following rigorous assessment.
- Treatment of comorbidities — in parallel with obesity treatment.
Many Swedish paediatric outpatient clinics use digital treatment support from Evira in their obesity care. Information about affiliated clinics is available at evira.se.
Where should one turn?
For children and adolescents: BVC (child health centre), the school health service, or the primary care centre provides a referral to BUMM, a paediatric clinic, or a specialised obesity unit. See assessment at BUMM.
For adults: the primary care centre can refer to a regional obesity clinic.
Points to consider
Class 3 obesity is a disease — not a matter of willpower. Early and appropriate treatment significantly reduces the risk of comorbidities. For children, treatment always centres on the entire family's habits, not on the individual child's body. See how to talk to children about overweight and obesity.
Related questions
- What is childhood obesity?Childhood obesity is an iso-BMI of 30 or higher — a chronic disease that should be treated within healthcare.
- What is childhood obesity?Childhood obesity is an iso-BMI of 30 or higher — a chronic disease that should be treated within healthcare.
- Ozempic and Wegovy for children — what applies?What are the rules for Wegovy, Saxenda, and Ozempic for children in Swedish healthcare? Clinical overview.
Sources
- WHO — BMI classification
- Socialstyrelsen — National guidelines for obesity care (2023)
- 1177 Vårdguiden — Obesity and overweight in children
Last updated: June 2026