Cultures of nasal or presternal swabs form part of the routine preoperative screening of patients on the cardiac surgical ward. During a trial of antibiotic prophylaxis in 314 patients, preoperative isolates of Staphylococcus aureus and coagulase-negative staphylococci were compared with strains associated with postoperative sternal wound breakdown (24 patients) and prosthetic valve endocarditis (3 patients). Morphology, antibiotic sensitivity pattern, plasmid analysis and phage typing were used to differentiate strains. In only three cases of wound infection and one of prosthetic valve endocarditis were pathogenic staphylococci not distinguishable from preoperative isolates. The collection of superficial swabs for this purpose before cardiac surgery is therefore unlikely to be cost effective.