To assess the familial aggregation of uterine myomas in Japanese women with myomas, one hundred forty four women requiring surgery for myomas and 288 age-matched healthy controls were studied in Hokkaido, Japan. The incidence of positive first-degree family history of myomas among women aged 45-54 years with myomas was greater than that among controls (31.5% versus 15.2%, respectively, p < 0.01). Analyses categorized by the status of parity and familiality among subjects showed that the risk for myomas was the greatest in women who had both fewer births (parity = 0 or 1) and the positive family history of myomas as compared with those who had both more births (parity > or = 2) and the negative familiality of myomas (odds ratio = 5.8, 95% confidence interval = 2.3 - 14.6). The results of this study suggest that Japanese middle-aged women with myomas have the familial predisposition of uterine myomas. Furthermore, nulliparous women with the familial aggregation of myomas may be at increased risk of the disease.