Endometriosis may appear as complex masses with low-level echoes from hemorrhage, as in this case. These masses are sometimes called "chocolate cysts." Endometriosis may also have a solid appearance, related to a large amount of clotted blood, or appear primarily cystic, as in the ultrasound study of another patient (Film 3).

The most frequent site of involvement of endometriosis is the ovary, although the cul-de- sac, uterus, tubes, bladder, and bowel can be involved.