Hemarthrosis is a bleeding into joint spaces. It is a common feature of hemophilia.

It usually follows injury but occurs mainly in patients with a predisposition to hemorrhage such as those being treated with warfarin (or other anticoagulants) and patients with hemophilia.^{[citation needed]}

It can be associated with knee joint arthroplasty.

It has also been reported as a part of hemorrhagic syndrome in the Crimean-Congo Hemorrhagic Fever, suggesting a viral cause to the bleeding in a joint space.

Hemarthrosis is diagnosed through the methods listed below:

A physical examination is the first step, with the joints of the patient moved and bent to study possible loss of functioning.

Synovial fluid analysis is another method to diagnose Hemarthrosis. It involves a small needle being inserted into the joint to draw the fluid. Reddish-colored hue of the sample is an indication of the blood being present. Imaging tests are normally done. The tests also include MRI, ultrasound and X-ray test, which give better information about the joint inflammation. Although MRI is superior method for this assessment, the US using the HEAD-US method performed by paediatric radiologists is a reliable tool for detection and quantification of haemophilic arthropathy in children in comparison to MRI.

In hemophilia it may occur spontaneously, and recurrent hemarthroses are a major cause of disability in that patient group due to hemophilic arthropathy, requiring synovectomy, joint replacement and increased medical therapy to prevent further bleeding episodes.