Approval by the US Food and Drug Administration (FDA) of a drug for a given indication is thought to reassure clinicians, other health care providers, and patients that substantial evidence of effectiveness exists for specific indicated populations (patients and diseases). This study examines whether FDA approval of certain antibiotics should be so reassuring for all patient populations identified in the FDA-approved labels. Specifically, this study compared patient populations covered by FDA-approved labels for 21 novel antibiotics approved between 1999 and 2018 to the patient exclusion and inclusion criteria of pivotal trials that supported those approvals. We found that every FDA-approved label for these antibiotics included at least one identifiable patient population that was explicitly excluded from enrolling in the supporting pivotal trials. Two antibiotics, bedaquiline and ceftazidime-avibactam, were approved for use in populations that were fully excluded from enrolling in registration trials.