Objective: To develop an equation for predicting the body density of sumo wrestlers. Methods: The following were measured: subcutaneous fat thickness measured at nine sites using B-mode ultrasound equipment; circumference at seven sites; hand to leg bioelectrical impedance. The subjects consisted of 24 college sumo wrestlers (mean age 19.7 years, mean body weight 111.2 kg) and 24 matched obese controls (mean age 19.1 years and mean body weight 111.2 kg). In addition, body density was measured by the underwater weighing method, and the percentage of fat was calculated from the measured body density. Results: Linear regression analysis was used to estimate the relation between body density and other variables, and a predictive equation for the body density was derived: y = 1.088-0.00036 x (fat thickness at nine sites) (r2 = 0.90) for the sumo wrestler group and y = 1.083-0.00033 x (fat thickness at nine sites) (r2 = 0.91) for the control group. A multiple regression analysis was performed using the body density as the objective variable, and other measured items as the explanatory variables. This was used to derive a predictive equation: y = 1.121-0.00038 x (fat thickness of abdomen)-0.00043 x (circumference of hips)-0.00142 x (fat thickness of triceps) (r2 = 0.94) for the sumo wrestler group, and y = 1.076-0.00070 x (fat thickness of abdomen)-0.00140 x (fat thickness of tibialis) (r2 = 0.91) for the control group. The difference between the two equations was due to the difference in body fat distribution. Neither of these predictive equations is applicable to non-overweight non-athletes. Conclusion: This is the first predictive equation developed for the body density of sumo wrestlers.