PURPOSE: To investigate the location of popliteal lymph nodes and the relationship between patient age and their number, size, and the occurrence of fatty chang. MATERIALS AND METHODS: We retrospectively evaluated the magnetic resonance (MR) images of 222 patients [age range, 8-79 (mean, 47.1) years]who had undergone MRI of the knee after its internal derangement. Images were obtained in the axial, coronal, and sagittal planes. A lymph node was defined as 'observed' if it was visible in at least two planes, such as axial and sagittal or axial and coronal. With regard to location, nodes were classified as anteromedial, anterolateral, posteromedial, or posterolateral, depending upon their relationship with the popliteal vein. To determine their size, the smallest diameter was measured. RESULTS: Popliteal lymph nodes were present in 116 of 222 examinations (52.3%), and their total number was 158 (mean, 1.36). Patients'age correlated negatively with their presence (R square=0.826), and positively with the occurrence of fatty change (R square=0.840). Sixty- five of 158 lymph nodes (41.1%) were located anteromedially, 58 (36.7%) posterolaterally, 27 (17.1%) anterolaterally, and eight (5.1%) posteromedially. Their distance from the most distal femoral articular surface was 4.6+/-1.4 cm (mean +/-SD), and their mean diameter was 4.96+/-2.4 mm (mean +/-SD; range, 4-8 mm). CONCLUSION: The number of popliteal lymph nodes decreased with age, while the incidence of fatty change increased. Nodes were most frequently anteromedial or posterilateral to the popliteal vein.