OBJECTIVE: To look for the frequency and the influence of apatite-like deposits in the synovial membrane of advanced osteoarthritic joints. METHODS: Synovium of 16 joints undergoing total arthroplasty for advanced primary osteoarthritis was embedded in paraffin. Adjacent sections were stained with alizarin red S, Mowat's pentachrome, Hematoxylin-eosin and Gomori to show apatite-like deposits, collagen, cartilage fragments, vessels, cells and iron respectively. Histomorphometry was carried out for the apatite-like and collagen deposits by the point counting method; the density of vessels and cells was also quantified. RESULTS: 14 out of the 16 specimens contained apatite-like material, mostly on the synovial surface or just beneath. There was no correlation between the apatite-like deposits and any other measured histological parameter (fibrosis, villus length, synovial lining cell width, density of synovial vessels or cells). Interestingly, the p value for a correlation between the amounts of apatite and collagen deposits was close to significance (r = 0.53; p = 0.055) when the relative volume of the apatite-like material was less than 1.2% (n = 13). CONCLUSIONS: Apatite-like deposits are frequently observed in the synovium of late stage osteoarthritis. Although not statistically significant, these results suggest a possible association between apatite-like and collagen deposits when the amount of apatite deposits is low. Further quantitative studies are recommended to investigate this observation.