Twenty cases of mediastinitis after coronary artery bypass graft operations in 1985-1987 were reviewed to determine risk factors. Two distinct clusters with a methicillin-resistant Staphylococcus aureus (MRSA) strain occurred in 1986. One resident was exposed to six cases but to only 5 of 24 controls (P less than .008). Cultures of his nares in January and November 1986 revealed the same MRSA strain as that of the cases. An attempt to eradicate the resident's nasal carriage of MRSA in January 1986 failed; eradication of his carrier state was achieved only after treatment with mupirocin. In a case-control study examining patients exposed to the resident, a prolonged duration of surgery (P less than .05) and a preoperative albumin level of less than 3.0 g/dl (P less than .009) were associated with mediastinitis with this MRSA. For the other 14 mediastinitis patients, who were not exposed to the resident, a preoperative albumin level of less than 3.0 g/dl was also a risk factor (8/14 cases vs. 8/43 controls, P less than .009). Thus, this study suggests that it is important to follow MRSA disseminators and to recognize that preoperative serum albumin levels are a risk factor for mediastinitis.