Increased use of antibiotics in any community increases the risks that future bacterial strains will resist the effects of current antibiotics. The consequences of a resistant bacterial strain include costs for more expensive and powerful drugs, additional hospital days, and on rare occasion, death. A key to understanding the importance of this problem is better knowledge about the rate that resistance increases and persists as antibiotic use rates increase. Using the scant evidence available in the literature, this study conducts a sensitivity analysis to calculate the unrecognized costs of antibiotic use annually in the United States under various possible circumstances. For the estimated 150 million annual antibiotic prescriptions, the unrecognized costs appears to be at least $.1 billion, and they may exceed $30 billion in the worst case. The estimates of the burden caused by bacterial resistance to antibiotics depend heavily on unknown parameters, including the rate that resistance occurs, the dose-resistance patterns through time, the frequency of inappropriate use of antibiotics, and the frequency with which death occurs due to a resistant bacterial infection. New studies in each of these areas are needed to improve our understanding of the extent of the resistance problem.