BACKGROUND: Adult females with menorrhagia may have unrecognized mild von Willebrand's disease. Most females with known von Willebrand's disease report menorrhagia. CASE: A 38-year-old healthy female desired contraception. She described heavy menses lasting 8 days since menarche. History included uncomplicated vaginal deliveries. Physical examination was normal. The levonorgestrel intrauterine system was placed. Her menorrhagia then worsened to 14 successive days each month. Evaluation revealed mild von Willebrand's disease. At 19 months of follow-up, she retained her levonorgestrel intrauterine system and avoided pregnancy. Her menses slowly improved to spotting for 8 days each month. CONCLUSION: Von Willebrand's disease is a likely cause of menorrhagia that goes unrecognized as well as a potential cause for "failed" conservative treatment for menorrhagia. Understanding the cause of menorrhagia is an important aspect of evaluating this common symptom.