Pulmonary hemorrhage and hemoptysis are the second-most common and potentially life-threatening complications after pneumothorax following percutaneous computed tomography-guided transthoracic lung biopsy (PCTLB). Preventing hemorrhagic complications after PCTLB requires an accurate estimation of risk factors. This study investigated the risk factors associated with pulmonary hemorrhage and hemoptysis following PCTLB, and whether the ratio of main pulmonary artery diameter (mPAD) to ascending aorta diameter (mPAD/AAD ratio) is a risk factor. We retrospectively analyzed 1,090 cases of PCTLB obtained from 1,050 patients using a core needle. The risk factors for overall pulmonary hemorrhage, higher-grade pulmonary hemorrhage, and hemoptysis were evaluated by multivariate analysis of patient characteristics, computed tomography (CT) imaging data including pulmonary artery diameter (mPAD) to ascending aorta diameter (mPAD/AAD) ratio, technical variables related to the biopsy, and pathologic findings. Pulmonary hemorrhage occurred in 31.38% (342/1,090) of PCTLB cases, including lower-grade (24.4%, 266/1,090) and higher-grade hemorrhage (6.97%, 76/1,090). The incidence of hemoptysis was 3.03% (33/1,090). Multivariate analysis revealed significant associations between overall pulmonary hemorrhage and lesion location in the lower lobe, subsolid and smaller lesions, greater lesion depth, and lung metastases. For higher-grade pulmonary hemorrhage, an mPAD/AAD ratio >1, smaller lesions, greater lesion depth, emphysema, and lung metastases were risk factors. Risk factors for hemoptysis were history of hypertension and lower- and higher-grade pulmonary hemorrhage. Pulmonary artery enlargement detected by CT (mPAD/AAD ratio >1) is independently associated with higher-grade pulmonary hemorrhage following PCTLB.