Historically thought of as a disease that affects adult women, endometriosis increasingly is being diagnosed in the adolescent population. This disorder, which was originally described more than a century ago, still represents a vague and perplexing entity that frequently results in chronic pelvic pain, adhesive disease, and infertility. The purpose of this Committee Opinion is to highlight the differences in adolescent and adult types of endometriosis. Early diagnosis and treatment during adolescence may decrease disease progression and prevent subsequent infertility.