Objectives: To investigate the degree of bacterial contamination in the sternal wound during cardiac surgery and the sternal skin flora after operation in order to increase our understanding of the pathogenesis of sternal wound infections. Design: Prospective study where cultures were taken peri- and postoperatively from sternal wounds and skin. Setting: University Hospital. Patients: 201 cardiac surgery patients. Results: 89% of the patients grew bacteria from the subcutaneous sternal tissue. 98% of the patients showed bacterial growth on the surrounding skin at the end of the operation. We found both commensal and nosocomial bacteria in the sternal wound. These bacteria had different temporal distribution patterns. Coagulase-negative staphylococci (CoNS) and Propionibacterium acnes (PA) were by far the most prevalent bacteria during and after the operation. Furthermore, 41% of patients had more than 10 000 CFU/pad CoNS on the skin. There was no correlation between length of operation and number of bacteria. Men displayed higher bacterial counts than women on the skin. Conclusion: Skin preparation with ethanol/chlorhexidine is unable to suppress the physiological skin flora for the duration of a heart operation. A decrease of CoNS and PA postoperatively can be caused by competitive recolonisation of commensal and nosocomial bacteria.