ABSTRACT This study analyzed the PGE2 synthesis of patients affected by secondary osteoarthritis of the knee in synovial tissues after He–Ne laser radiation in vitro. Patients ranged in age from 58 to 77 years old. Eight female patients were studied. A biopsy was done before and other after the laser treatment. Each patient received 15 applications of He–Ne laser with an energy density of 8 J/cm2 each. We used the test for paired data:d = 5.25 ± 2.32 ng/mg of tissue/60 min (p < 0.01). We concluded that the He–Ne laser decreases PGE2 synthesis in vitro in patients with osteoarthritis of the knee. This effect may cause the attenuation of the inflammatory process in which PGE2 participates.