156 patients undergoing primary bilateral or unilateral total hip arthroplasty were randomized to receive either a cemented or cementless stem. Arterial blood pressure, right atrial pressure, arterial oxygen tension, and carbon dioxide tension were assessed at up to 48 hours postoperatively. The presence of lipid and cellular contents of bone marrow was determined. The prevalence of fat embolism and bone marrow-cell embolization was not significantly different between the bilateral and unilateral groups or between the cemented and cementless stem groups. Patients with bone marrow-cell embolization had a significantly lower arterial oxygen tension and oxygen saturation than did patients without embolization.