Abstract: Objective To investigate the distribution of ultrasound (US)-detected knee joint effusion in arthritis patients. Methods Lateral, midline and medial regions of suprapatellar pouch effusion and its depth, length, cross section area in arthritis patients admitted to our department from July 2012 to December 2012 were detected by high resolution ultrasound examination. Results Thirty-eight arthritis patients with 68 knee joints involved were enrolled in this study. The maximal effusion depth and length were significantly higher in lateral and midline regions of knee joint than in medial region of knee joint (4.8±4.5) mm and (4.5±4.3)mm vs (3.1±4.3) mm, (P＜ 0.001); (26.5±18.1) mm and (28.5±19.7) mm vs (18.7±20.5) mm, (P＜ 0.001). No significant difference was observed in effusion depth and length between lateral and midline regions of knee joint (P＞ 0.05). The amount of effusion was significantly greater in lateral and midline regions of knee joint in those with a small amount of effusion (P＜ 0.001). No significant difference was found in the effusion in middle and medial regions of knee joint in those with a large amount of effusion(P=0.133). The amount of effusion was significantly greater in those with rheumatoid arthritis than in those with spondyloarthritis or osteoarthritis (405.6±325.4) mm² vs (213.5±241.6) mm² vs (74.6±72.3) mm², (P=0.004, P=0.005). Conclusion A small effusion is mainly distributed in the lateral and midline regions of knee joint. Lateral and midline regions of knee joint are the first choice of knee joint puncture, which contributes to its success.