Purpose Aspiration pneumonia (AP) has significant incidence and impact on mortality. However, data about clinical diagnosis criteria are scarce. We aimed to evaluate according to predefined criteria the prevalence of true AP and its impact on antibiotic stewardship. Methods Retrospective study of patients whose main diagnosis was AP hospitalized at Amiens University Hospital in 2018. We first defined diagnostic criteria of certainty for pneumonia and aspiration. AP was then classified according to degree of certainty. Results Among 862 cases of AP, its diagnosis was certain, likely, probably in excess, certainly in excess or absent in 2 % (n = 17), 3 % (n = 26), 50.5 % (n = 433), 23.1 % (n = 198) and 21.4 % (n = 183) respectively. Irrelevant use of amoxicillin-clavulanic acid and metronidazole was found in 27 % and 13 % of cases, respectively. Conclusions The diagnosis of AP is frequently excessive, and diagnostic tools are urgently needed to improve antibiotic stewardship.