Vascularity is said to be increased (subjectively) in rheumatoid arthritis, yet synovial fluid is hypoxic and acidotic. Morphometry by image analysis was used to quantify vascularity in normal and rheumatoid synovium from the knee joint. Capillaries were distributed more deeply in the rheumatoid synovium (93.3 microns, compared with 32.5 microns in normal synovium) and were significantly less densely arranged (80.2/mm2 rheumatoid, 241.5/mm2 normal). The blood volume fraction fell from 2.9% in normal knee synovium to 1.2% in rheumatoid synovium. These results imply that there is impairment of O2 transfer, which contributes to the intraarticular hypoxia, in rheumatoid arthritis.