Elbow bursitis, also called olecranon bursitis, causes fluid to collect in a sac that lies behind the elbow, called the olecranon bursa. Elbow bursitis is the most common type of bursitis. A bursa is a slippery, sac-like tissue that normally allows smooth movement around bony prominences, such as the point behind the elbow. When a bursa becomes inflamed, the sac fills with fluid. This can cause pain and a noticeable swelling behind the elbow. The olecranon bursa measures up to 6 cm in length, and can swell to a size larger than a golf ball. Elbow bursitis may follow a traumatic accident, such as a falling on the back of the elbow, or it may seemingly pop up out of nowhere.