Ultrasonography is convenient for patients because it is non-invasive and mobile. We report two cases of knee disease detected by ultrasonography. Case 1, was a seventeen-year-old girl. She had suffered from increasing right knee pain for one year. Arthroscopy detected a lateral meniscal rupture, but a meniscal cyst was not diagnosed until ultrasonography was performed. The meniscus was resected and the cyst aspirated arthroscopically. Case 2, was a 57-year-old woman. Hemoarthrosis of her left knee appeared with no history of trauma. Ultrasonography detected an anechoic mass, but no abnormalities were detected arthroscopically. After arthroscopy, MRI confirmed a tumor in the region where the ultrasonograph had previously detected it. The tumor was resected and its histological diagnosis was inflammatory granulation. Ultrasonography is a useful screening examination for the knee.