Inflammation has been suggested as a component in the osteoarthritic process. We have examined 150 knees at autopsy to correlate the degree of osteoarthritis (OA) with synovial inflammation, and with radiographic or pathologic evidence of calcification as a possible source of crystal induced inflammation. Roentgenographic evidence of calcification and histologic finding of synovial reaction (proliferation or inflammation) were seen more frequently (p less than .05) in the knees of patients with more advanced OA. Perichondrocytic staining for calcium and phosphorus was seen in 93% of patients with severe OA and in only 24% of those with normal or minimal OA. Calcium hydroxyapatite crystals were identified by electron microscopy in perichondrocytic and other areas in 8 of 15 cartilage samples with moderate to severe OA and in none with normal appearing cartilage. In advanced OA, there were bone fragments embedded in cartilage and synovium. Although the pathogenetic sequences are uncertain, we speculate that calcium hydroxyapatite (and pyrophosphate) crystals and/or bone shards can contribute to both synovitis and cartilage degeneration.