Initial staging by positron emission tomography/computed tomography (PET/CT) scanning is recommended for patients with diffuse large B-cell lymphoma (DLBCL). Whether both PET/CT and bone marrow biopsy (BMB) are required remains unclear. This study examined whether staging by PET/CT is sufficient. Participants with untreated DLBCL assessed using both PET/CT and BMB were included. Patients received independent diagnostic assessments from a radiologist and a hematopathologist. Both hematoxylin–eosin staining and CD20 immunostaining were performed to determine the bone marrow involvement in BMB. A total of 84 patients were included. The number of patients with positive bone marrow involvement identified by PET/CT and BMB was 16 (19%) and 22 (26%), respectively. Eight (10%) patients showed positive results in both tests. When considering BMB as a reference, PET/CT showed 36% sensitivity and 87% specificity, with positive and negative predictive values of 50% and 79%, respectively. BMB-positive patients had shorter progression-free (PFS) and overall (OS) survival than their BMB-negative counterparts. Compared to PET/CT-negative patients, patients with positive results did not show any significant differences in PFS and OS. However, among 16 PET/CT-positive patients, poor PFS and OS were observed among patients who were also BMB positive. BMB remains a mandatory step in staging of untreated DLBCL patients.