The purpose of this study was to know the pathology associated with Baker's cyst (BC) in a rheumatology clinic and to evaluate the incidence, characteristics, and complications of BC. We reviewed the rheumatology ultrasound laboratory charts of patients with BC from Oct 2006 through Dec 2008. Demographic and disease data were also collected. Of the 1,120 patients who underwent ultrasound studies, 145 (12.9%) were found to have 180 BCs. The associated diseases were as follows: 91 (50.6%) osteoarthritis (OA) of the knee, 37 (20.6%) rheumatoid arthritis (RA), 25 (13.9%) gout, 14 (7.8%) seronegative spondyloarthropathy (SpA), and 13 (7.2%) pyrophosphate arthropathy. We found ruptured BCs in 12 patients, whose associated pathologies were in the following: four RA, four OA of the knee, two gout, one SpA, and one pyrophosphate arthropathy. The most frequent associated arthropathy of BCs was OA (50.6%), followed by RA (20.6%). However, in the cases of ruptured BC, the inflammatory pathology (66.7%) is more frequent than the degenerative one (33.3%).