Patients with hydrarthrosis of the knee, whatever its origin, always exhibit a change in synovial permeability. This can be detected by scintigraphy (visualization of the site and extent of the hydrarthrosis) and scintimetric investigation. In the present study, patients with hydrarthrosis of the knee of a degenerative, inflammatory or traumatic origin, received an appropriate dose of 99mTc pertechnetate, and a gamma camera was used as the detector. At the same time a scintimetric study was carried out to provide a quantitative assessment of the rate of absorption of the radio-isotope. This indicates the functional state of the synovial membrane. The results of these investigations are of value both in diagnosis and in treatment.