The gallbladder and biliary system were scanned by real-time ultrasound in 2274 patients undergoing diagnostic ultrasound for other than gallbladder disease. Patients (212) with previous gallbladder symptoms or with nonvisualization of their gallbladder were excluded from the study. Obesity is the most important risk factor for the development of asymptomatic gallstones in women (P less than .01), although it is not a significant factor in men. Increasing age is an important risk factor for both sexes, and the only significant risk factor in men (P less than .01). While few men have asymptomatic gallstones before the age of 40, 5% of women aged 20-29 and 9% aged 30-39 do. In the 40 and over age group, men (14%) and women (11%) had insignificantly different prevalences. In women, the number of previous pregnancies is a significant risk factor (chi-square = 5.4, P = .02). For instance, there is a 3%, 8%, and 17% overall frequency of gallstones in women with 0, 3, and 6 or more previous pregnancies, respectively. A stepwise logistic regression analysis, after adjusting for age, body mass index, and for women, number of pregnancies, found no significant increased risk related to race (P = .40), high blood pressure (P = .43), heart disease (P = .47), or diabetes (P = .46). After age adjustment, there is no significant gender effect (P = .25). Asymptomatic gallstones are a relatively common occurrence in men over 40 and women over 30. While age is the only significant risk factor in men, obesity, parity, and to a lesser extent age were significant risk factors in women.