To determine whether contaminant blood cultures increase resource utilization, we studied charge and length of stay data for episodes in which blood cultures were obtained from hospitalized adults. Compared with 1097 negative episodes, 94 false-positive episodes were associated with increased subsequent length of stay (median, 12.5 vs 8 days) and subsequent total charges (median, $13 116 vs $8731), pharmacy charges (median, $1456 vs $798), and laboratory charges (median, $2057 vs $1426). In multivariate analyses, contaminants were independently correlated with 20% and 39% increases in total subsequent laboratory charges and intravenous antibiotic charges, respectively. Thus, the true costs of contaminants may greatly exceed those of the test itself. Identifying patients at very low risk of bacteremia and attention to sterile technique may reduce costs by decreasing the frequency of contaminants.