Ultrasonographic findings in patients with diseases of the appendix, including acute appendicitis, suspected appendiceal abscess, and palpable right-lower-quadrant abdominal mass, are described. An appendiceal abscess may manifest as a cystic mass, a mixed solid and cystic mass, or a hypoechoic solid mass. An appendiceal calculus within an abscess can be recognized as a hyperechoic structure with acoustic shadowing. On ultrasonography, acute appendicitis in female patients may mimic tubo-ovarian disease. Ultrasonographic features of isolated Crohn's disease of the appendix and mucocele of the appendix are also described.