Three cases of systemic reaction to bone cement are reported. Two patients had a cardiac arrest immediately after insertion of the cement during total hip replacement. The third patient had bilateral total hip replacement, the second procedure being done six weeks after the first. On each occasion the patient developed unexplained pyrexia and it is suggested that this might be due to allergy. The chemistry of bone cement is discussed and it is suggested that absorbed monomer may be responsible for systemic reactions to the cement.