The yield of blood cultures depends on the volume of blood cultured. We recently discovered that 15% of blood-culture specimens from adults in our hospital were being collected in 3.5-mL pediatric tubes and that another 5%, drawn in 10-mL adult tubes, contained less than 5 mL of blood. A comparison of 829 matched pairs of standard-volume (mean, 8.7 mL) and low-volume (mean, 2.7 mL) blood cultures showed that standard-volume cultures had a substantially higher detection rate for bloodstream infection than did low-volume cultures (92% compared with 69%; difference, 23% [95% CI, 9% to 37%]; P < 0.001). Our data, together with an analysis of previous studies, show that the yield of blood cultures in adults increases approximately 3% per millilitre of blood cultured. A survey of 158 U.S. clinical microbiology laboratory directors in the American Society of Clinical Pathologists showed that only 20% of 71 responding laboratories record the volume of blood submitted for culture and that the practice of culturing suboptimal volumes of blood from adults is widespread. Clinical laboratories should routinely monitor the volume of blood cultured as a quality-assurance measure. Blood-culture specimens from adults should not be drawn using small pediatric tubes.