We retrospectively studied the use of endoscopic ultrasound–guided fine-needle aspiration biopsy (EUS-FNAB) as a tool for the diagnosis of deep-seated lymphoma. An on-site assessment at the time of EUS-FNAB was performed by a cytopathologist using Diff-Quik (American Scientific Products, McGraw Park, IL) stain. In addition, Papanicolaou stains were performed on EUS-FNAB smears, immunohistochemical stains were performed on cell blocks, and additional samples were sent for flow cytometric analysis. Final cytologic diagnosis was correlated with surgical pathology and/or clinical follow-up. We evaluated EUS-FNAB specimens of deep-seated lymph nodes, spleen, stomach, and pancreas, and 1 EUS-guided needle core biopsy specimen of a lymph node. Thirteen cases of deep-seated lymphoma were diagnosed, including non-Hodgkin lymphomas and Hodgkin lymphoma. One case of hairy cell leukemia was diagnosed. EUS-FNAB is a minimally invasive, cost-effective, and useful tool for the primary diagnosis or staging of deep-seated lymphomas.