In order to evaluate ultrasonography in patients with suspected hip joint effusions, 123 consecutive patients were examined prospectively. Twenty healthy subjects were used as a control group. The normal sonoanatomy, the recommended scanning approach, and the diagnostic features of intra-articular joint effusions are presented. Even minor fluid collections of 1 or 2 ml could be accurately detected. Transient synovitis and fresh hemorrhagic effusions proved to be echofree, whereas clotted hemorrhagic collections or septic arthritis showed non-echofree effusions. Based on intraoperative and computed tomography (CT) data, ultrasonography is clearly superior to X-ray films in detecting joint effusions. A negative sonogram will exclude a fluid collection; the depiction of an echofree effusion will virtually rule out septic arthritis. The use of additional imaging techniques can be reduced greatly, as ultrasonography seems to be an improved method for the early diagnosis of septic arthritis.