The nature of 103 intra-abdominal fluid collections examined by ultrasound was determined by autopsy, needle aspiration, spontaneous drainage or surgical exploration. Abscesses, chronic hematomas, lymphoceles, urinomas, cysts and fluid-filled bowel loops are all approximately elliptical or circular in cross section. Collections of ascites have a very irregular outline. Bowel loops appear as multiple circles, all of similar size. Chronic hematomas contain strongly echoing material, while lymphoceles, urinomas, renal and hepatic cysts and collections of ascites are echo-free. Ovarian cysts into which there has been bleeding may contain echoes. The walls of abscesses, chronic hematomas, and bowel loops are less sharp than the walls of other collections. Intrahepatic abscesses may have more ragged walls than abscesses elsewhere. Ultrasonic characteristics of abscesses and hematomas overlap. It is sometimes possible to distinguish the nature of the fluid by its ultrasonic properties, but atypical appearances are not rare.