While synovitis is common in advanced osteoarthritis (OA), its prevalence and severity in patients with early or mild OA are uncertain. In our study synovial biopsies from patients with arthroscopic evidence of OA whose radiographs were normal, or showed only mild/moderate changes of OA, were examined to determine the prevalence and severity of lining cell proliferation and mononuclear cell infiltration. Synovitis was present in only 16 of 29 patients (55%) who underwent arthroscopy because of chronic knee pain and were found to have OA; no synovium from 50% of the 22 patients in this group with full thickness cartilage ulceration showed infiltration with mononuclear cells. Similarly, no evidence of synovitis was seen in biopsies from 7 of 14 additional patients with OA who did not have knee pain but who underwent arthroscopy to evaluate joint instability. An association was seen between synovial mononuclear cell infiltration and thickness of the synovial lining cell layer (p less than 0.03), but lining cell hyperplasia was found in samples from only 12% of the patients with OA in our series. The severity of OA cartilage lesions was unrelated to severity of synovitis and no topographic relationship was found between cartilage ulceration and synovitis.