Hemarthrosis increases synovial cell proliferation. The right knees of hemophilic mice were subjected to a controlled blunt trauma to induce hemarthrosis. The animals were killed on day 7 after the initial injury, on day 14 after 2 injuries on days 0 and 7, or on day 21 after 3 injuries on days 0, 7, and 14 to simulate recurrent target joint bleeding. Equatorial histologic sections through the knee joints were prepared and examined by light microscopy. Five areas of the synovial membrane were chosen randomly and the number of synovial cell layers counted by 4 independent observers blinded to the experimental conditions. The results were pooled and the average and standard deviation (shown by error bars) calculated. Student t test was used to compare the differences between groups.