The goal of this study is to determine and compare the β-endorphin levels in the synovial fluid of patients with different joint disorders (avascular necrosis, AVN; osteoarthritis, OA; and rheumatoid arthritis, RA of the hip or knee). Eighty-seven patients were involved in our study with an average age of 62 years. Thirty-three patients had AVN (18 hips, 15 knees). Twenty-three patients were diagnosed with OA (14 hips, 9 knees), and 31 patients suffered from RA (12 hips, 19 knees). We measured the β-endorphin levels of the synovial fluids -harvested from surgery-with radioimmunoassay. No significant difference was found in the β-endorphin levels of the synovial fluid from AVN comparing to OA and RA, however β-endorphin level was significantly higher in RA group than among patients with OA (p = 0.01). Synovial β-endorphin level was slightly lower in knee comparing to hip joint p = (0.06). When examining the different joints separately in compliance with diagnoses, we concluded that the synovial β-endorphin level from AVN was between the values of OA and RA without significant difference, whereas it was significantly higher in the knee of RA, than of OA groups (p = 0.05 knee, p = 0.2 hip). Our results confirmed those experiments which stated that there is a significant increase in synovial β-endorphin level in patients with inflammatory autoimmune diseases (e.g., RA), comparing to the level measured in degenerative conditions (e.g., OA).