Periarticular heterotopic bone (HB) formation after hip arthroplasty can be prevented by indomethacin or radiation. It is therefore important to identify patients at risk. The authors analyzed 124 consecutive cases of primary total hip arthroplasty (THA) performed by the same trans-trochanteric operative approach in 124 patients. Heterotopic bone occurred more often in men (84%) than in women (67%), and greater amounts of bone were formed in men. On anteroposterior (AP) roentgenographs, the median area of projected HB in men was 24.7 cm2 and 0.69 cm2 in women. Heterotopic bone was less common in patients with polyarthritis compared with patients with osteoarthrosis. In women with osteoarthrosis, old age was associated with the occurrence of HB; women older than 65 years of age had 0.70 cm2 of HB on AP roentgenographs (mean), whereas those younger than 65 years had 0.18 cm2. Hypertrophic osteoarthrosis was more common in men, but associated with HB only in women. Women with hypertrophic osteoarthrosis developed the same amount of HB as men with osteoarthrosis. Men with osteoarthrosis or sequelae after fractures, older women with osteoarthrosis, and women with hypertrophic osteoarthrosis should be considered for prophylactic treatment against HB formation after a THA.