The aim of this study was to determine the threshold for detecting knee effusion by ultrasound (US). Five knee specimens from embalmed cadavers were studied. Intra-articular injection of saline, blood and synovial fluid was performed under ultrasound control and methylene blue dye instillation. The smallest amount of fluid detectable by US in the knee was 7.4 ml for synovial fluid, 10.1 and 10.4 ml for saline solution and 9.7 for blood. The threshold for detecting knee joint effusion by US in cadaver specimens was 10 ml for saline and blood and 7 ml for synovial fluid.