ObjectivesPatients with vascular anomalies (VAs) who receive oral sirolimus may be at high risk of infectious complications. Antibiotic prophylaxis with trimethoprim-sulfamethoxazole (TMP-SMZ) has been advocated. However, there have been few evidence-based analyses on this topic. This study assessed the effect of prophylactic TMP-SMZ on the incidence of infections in VA patients receiving sirolimus monotherapy.MethodsA retrospective, multicenter chart review was performed on all VA patients receiving sirolimus treatment from August, 2013 to January, 2021.ResultsBefore January 2017, 112 patients were treated with sirolimus without antibiotic prophylaxis. In the subsequent period, 195 patients were treated with TMP-SMZ for at least 12 months during sirolimus therapy. The percentage of patients with at least one serious infection during the initial 12 months of sirolimus treatment did not differ between the groups (difference, 1.1%; 95% CI − 7.0–8.0%). We observed no difference in the incidence of individual infection or total adverse events between the groups. The rate of sirolimus discontinuation due to adverse events did not differ significantly between groups.ConclusionsWe demonstrated that prophylactic TMP-SMZ does not decrease the incidence of infection or improve tolerance in VA patients receiving sirolimus monotherapy.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13023-023-02740-3.