The formation of periarticular heterotopic bone after total hip arthroplasty is a frequent complication. The reported occurrences concerning this complication vary considerably in different reports, ranging from 15% to 90% with significant amounts in 1%-27% of the cases. Heterotopic ossification (HO) starts with the surgical operation, and the extent is well delineated on roentgenograms after six to 12 weeks. The amount of bone varies from small islands in the soft tissue to widespread bridging ossification. The cause of HO seems to be mainly related to systemic factors and is chiefly dependent on gender, diagnosis, and concurrent antiinflammatory medication. Patients at risk seem to be those with HO after a previous surgical operation, patients suffering from certain types of ankylosing spondylitis, otherwise healthy men with osteoarthrosis, patients with hypertrophic osteoarthrosis, and patients operated upon for fresh fractures or other posttraumatic conditions. The surgical technique and the local tissue trauma probably moderate both the occurrence and amount of HO. HO does not seem to cause pain or to decrease hip muscle strength but does limit hip mobility in cases with significant amount of ossification.