Synovial fluid ferritin levels in patients with traumatic hemarthrosis (HA), rheumatoid arthritis (RA), and osteoarthritis (OA) were measured by double antibody radioimmunoassay. Synovial fluid ferritin levels were significantly higher in 60 patients with HA (mean +/- S.D., 536 +/- 536 ng/ml) and 39 patients with RA (614 +/- 486 ng/ml) than in 20 patients with OA (130 +/- 119 ng/ml) (p < 0.01). Individual levels, however, considerably varied. In HA patients, the synovial fluid ferritin level correlated well with the duration of hemarthrosis, but not with hemoglobin, hematocrit, or an inflammatory synovial fluid index such as the leukocyte count. In RA patients, there was no significant correlation between the synovial fluid ferritin levels and any inflammatory parameter, such as catalase activity, synovial leukocyte counts (including polymorphs and monocytes) or the duration of arthritis. Our results indicate that the synovial fluid ferritin level reflects primarily hemoglobin degradation and appears unrelated to inflammation in joint diseases.