OBJECTIVE: Uterine leiomyoma, or fibroid tumors, are the leading indication for hysterectomy in the United States, but the proportion of women in whom fibroid tumors develop is not known. This study screened for fibroid tumors, independently of clinical symptoms, to estimate the age-specific proportion of black and white women in whom fibroid tumors develop. STUDY DESIGN: Randomly selected members of an urban health plan who were 35 to 49 years old participated (n = 1364 women). Medical records and self-report were used to assess fibroid status for those women who were no longer menstruating (most of whom had had hysterectomies). Premenopausal women were screened by ultrasonography. We estimated the age-specific cumulative incidence of fibroid tumors for black and white women. RESULTS: Thirty-five percent of premenopausal women had a previous diagnosis of fibroid tumors. Fifty-one percent of the premenopausal women who had no previous diagnosis had ultrasound evidence of fibroid tumors. The estimated cumulative incidence of tumors by age 50 was >80% for black women and nearly 70% for white women. The difference between the age-specific cumulative incidence curves for black and white women was highly significant (odds ratio, 2.9; 95% CI, 2.5-3.4; P <.001). CONCLUSION: The results of this study suggest that most black and white women in the United States develop uterine fibroid tumors before menopause and that uterine fibroid tumors develop in black women at earlier ages than in white women.