The aim of this study is:

1. To describe the natural course of acute cholecystitis, by performing repeated ultrasound examinations from day of admission to hospital until day of surgery or discharge.
2. To investigate if there is anything in the ultrasound picture that can predict a difficult operation.

Patients will be prospectively enrolled. The ultrasound picture (ultrasound variables: gallbladder volume, gallbladder wall thickness, and presence of oedema in the gallbladder wall), will be compared to clinical variables (tenderness in right upper quadrant, WBC, CRP and temperature) and to the time (hours) from onset of symptoms. Statistical analyses will be made to see if statistically significant, and clinically relevant, associations between variables exist.

Furthermore, for participants receiving acute surgery, an upper GI surgeon will evaluate the operation as easy, intermediate or difficult. In extension, investigators will look at associations between difficult surgery and all the variables previously mentioned (both ultrasound variables, clinical variables and time).

Patients admitted to Stockholm South General Hospital's Surgery Department, with a confirmed diagnosis of acute cholecystitis (according to the Tokyo Guidelines 2013) will be considered eligible for the study and asked to participate. Informed consent will be obtained from all participants. Participants will be examined with ultrasound of the gallbladder daily, while receiving regular care for acute cholecystitis, which means either waiting for acute surgery or being treated conservatively for as long as inpatient care is needed.

The ultrasound examinations will be performed by a radiologist or a specialised certified ultrasound trained nurse as well as by an ultrasound trained surgeon. Measures of gallbladder volume (measured in cm3), gallbladder wall thickness (measured in mm:s) and presence of oedema in the gallbladder wall (noted as a binomial variable 'yes/no') are recorded in a case report form (no.1) together with body temperature, labs and information about onset of symptoms.

The operating surgeon (a specialist in surgery) will evaluate the operation and grade it from easy to difficult and fill in a case report form (no.2).

Data will be collected from the forms and analysed using a regression model (generalised estimated equation or a mixed model).

Inclusion Criteria:

• Acute cholecystitis according to the Tokyo Guidelines 2013 criteria

Exclusion Criteria:

• Difficulties to communicate

Patients admitted to the hospital, with confirmed acute cholecystitis according to the Tokyo Guidelines 2013 will be asked to participate in the study