This study investigated the concentration of tartrate resistance acid phosphatase (TRAP) in the stifle synovial fluid of normal and osteoarthritis secondary to experimental medial patellar luxation (MPL). MPL was surgically produced in the left stifle (index) of 20 mixed small breed dogs (age 1 to 10 years, weight 2.8 to 10 kg) by placing purse string sutures around the parapatellar fibrocartilage and anchoring the patella with the fabellar ligament, and by medial imbrication and lateral release. The animals were randomly allocated in 2 groups; sham group (n=10), the right stifle was sham operated and control group (n=10). Radiographs were taken and synovial fluid was aspired from both stifles preoperatively and postoperatively at every 1.5 month intervals. One dog was euthanatized at every 1.5 month and tissue samples from both stifles were collected for histopathology. TRAP assay was performed using a biochemical assay in 96-well plates with p-nitrophenylphosphate (pNPP) as substrate. The clinical signs of osteoarthritis were obvious in the experimental dogs by 12 weeks of the surgical induction of MPL which was also evidenced in histopathology of the tissue samples. The concentration of TRAP in the index stifles significantly increased after 3 months as compared with that of the sham and control stifles. The surgically made MPL can be a tool for experimental induction of osteoarthritis. The TRAP assay can play an important role for the diagnosis and therapy of osteoarthritis in the dog.