Uterine leiomyomas, commonly called fibroids, are a major health concern for women of reproductive age. The objectives of the study described herein are to investigate the growth dynamics of uterine leiomyomas in a clinically relevant population of women. We will test the hypotheses that uterine leiomyomas are heterogeneous in terms of their growth characteristics and in their clinical symptoms or outcomes, and that differences in leiomyoma growth dynamics can be discriminated by molecular markers and cellular phenotypes. Participants will include 300 premenopausal women (greater than 18 years old) with at least one uterine leiomyoma. The inclusion criteria for patient enrollment is confirmed diagnosis of leiomyoma by ultrasound. At least one leiomyoma must be equal to or greater than 2 cm in diameter and the uterus must be enlarged to the size typical during the eigth week of pregnancy. After enrollment and informed consent, T1- and T2-weighted magnetic resonance image (MRI) scans will be conducted beginning at the first visit and then at 3, 6, and 12 months. Each patient will have a physical exam, provide urine and blood samples at each MRI visit, and respond to an initial extensive telephone-administered questionnaire followed by abbreviated monthly questionnaire updates. A number of the enrolled women will require surgical intervention (hysterectomy/myomectomy) as standard care. If surgery is an outcome for women enrolled in the study, MRI will be conducted before surgery and the surgical pathologist will map uterine leiomyomas for comparison to MRI. Leiomyoma samples will be analyzed for histopathological and molecular changes correlated with growth. Because hysterectomy and myomectomy are common outcomes in women with leiomyomas, we anticipate tissue will be available from at least 100 of the 300 women in the study. For those women who opt for surgery, we will also administer a brief (less than 5 minute) questionnaire clarifying their reason for electing surgery. Upon completion of data collection, we will be able to compare leiomyoma growth as a function of multiplicity and location; examine the relationship between leiomyoma growth and clinical symptoms or outcome; identify molecular, cellular, and pathological characteristics of leiomyomas with differing growth dynamics; and examine endocrinological parameters and lifestyle factors related to differential growth dynamics of uterine leiomyomas. The data may be used to establish a clinical severity scale and establish diagnostic markers currently not available for uterine leiomyomas.