Ten cases of endometriosis presenting as pelvic masses are reviewed. Ultrasonically, these lesions were classified into 3 categories: cystic, mixed and solid. In the cystic pattern, none had the smooth, well-defined wall and totally echo-free appearance characteristic of a simple cyst. Instead, the wall was shaggy and somewhat irregular with some evidence of septation. In the mixed type, the echographic appearance was compatible with pelvic inflammatory disease of an infectious etiology. In 3 cases with the solid pattern, the ultrasound appearance suggested ovarian malignancy. When the clinical setting and the age of the patient are suggestive of endometriosis, the sonographer should consider this possibility in his differential diagnosis of pelvic masses, regardless of the consistency of the lesion.