A 4-year retrospective study ascertained the relation between the magnitude of Staphylococcus aureus bacteremia and mortality. 253 episodes of S. aureus bacteremia diagnosed in a Danish regional Department of Clinical Microbiology were included. Blood cultures were routinely done by inoculation of aseptically drawn blood into multiple tubes of bacteriological media. The rate of positive tubes was assumed to distinguish high- from low-grade bacteremia without providing an absolute count of colony forming units. The median number of tubes inoculated was 18 (range 6-48) with approximately 0.5-1 ml of blood per tube. The rate of positive tubes was related neither to the source of infection, i.e. whether the S. aureus infection was nosocomially or community-acquired, nor to the portal of entry. However, mortality increased with increasing rate: it was 18% for rates < 0.30, 23% for 0.30-0.69, 30% for 0.70-0.99, and 37% for a rate of 1.0. Thus, for S. aureus infections the magnitude of bacteremia seems to be a prognostic factor with possible bearings on clinical and therapeutic decisions.