BACKGROUND: Infectious complications after endobronchial ultrasound-guided transbronchial biopsy with a guide sheath (EBUS-GS-TBB) are serious in that they may delay or change scheduled subsequent therapy. The aim of this study was to identify risk factors for infection after EBUS-GS-TBB. RESEARCH QUESTION: What are the risk factors for infection after EBUS-GS-TBB? STUDY DESIGN AND METHODS: We retrospectively reviewed the medical records of 1,045 consecutive patients who had undergone EBUS-GS-TBB for peripheral lung lesions between January 2013 and December 2017 at Fujita Health University Hospital. We evaluated the following risk factors for infectious complications after EBUS-GS-TBB: relevant patient characteristics (age and comorbidities), lesion size, CT scan features of target lesion (intratumoral low-density areas [LDAs] and cavitation), stenosis of responsible bronchus observed by bronchoscopy, and laboratory data before EBUS-GS-TBB (WBC count and C-reactive protein concentration). RESULTS: Forty-seven of the study patients developed infectious complications (24 with pneumonia, 14 with intratumoral infection, three with lung abscess, three with pleuritis, and three with empyema), among whom the complication caused a delay in cancer treatment in 13 patients, cancellation of cancer treatment in seven patients, and death in three patients. Multivariate analysis showed that cavitation (P = .007), intratumoral LDAs (P < .001), and stenosis of responsible bronchus observed by bronchoscopy (P < .001) were significantly associated with infectious complications after EBUS-GS-TBB. Prophylactic antibiotics had been administered to 13 patients in the infection group. Propensity matched analysis could not show significant benefit of prophylactic antibiotics in preventing post-EBUS-GS-TBB infections. INTERPRETATION: Cavitation, LDAs for CT scan features of target lesions, and stenosis of responsible bronchus observed by bronchoscopy are risk factors of post-EBUS-GS-TBB infection. In the cohort, prophylactic antibiotics failed to prevent infectious complications.