The most important determinant of outcome of a hip infection is the delay between the onset of the infection and treatment. Transient synovitis, and septic and tubercular arthritis of the hip remain common diagnostic problems. Conventional radiographic examinations are of little help in early diagnosis. Computed tomography, scintigraphy and magnetic resonance imaging, though informative, are expensive and not universally available. Ultrasonography (US) of the hip was done in 50 patients to define sonographic anatomy, detect joint effusions and correlate sonographic features with the etiology of the disease. Even small collections of fluid could be detected with accuracy. Both hypo- and hyperechoic synovial fluid was seen in septic arthritis, but hyperechoicity and a thickened capsule were the most characteristic findings. Synovial fluid had mixed echogenicity in tubercular and transient synovitis. The use of other (invasive) imaging modalities can be minimized because US can be used not only to demonstrate effusions early in the disease but also the status of the intra-articular compartment, joint capsule, bony surface and adjacent soft tissues. Ultrasonography should be used more commonly to diagnose infective arthritis, and no patient should be subjected to arthrotomy or drainage if US has ruled out the presence of a fluid collection.