OBJECTIVES--To provide prediction equations for balance measured as sway speed, a population of randomly selected subjects was used as a model. METHODS--265 subjects were selected from voter registration rolls in Arizona and Louisiana, USA. For validation these predicted values were compared with observed values obtained from a second population in California, USA. RESULTS--Sway speed with eyes open as a natural logarithm was: 1n (open) = -0.248333 -0.009634 x age +0.000164 x age2 and sway speed with eyes closed was: 1n (closed) = -0.585707 -0.023074 x age +0.000315 x age2 + 0.038496 x height (cm) CONCLUSIONS--The natural logarithm of balance with the eyes open was related to age by both linear and quadratic functions. Additionally with the eyes closed balance was dependent on height. These equations can be used to predict performance of groups being studied for effects of noise and chemicals. To define abnormality of individual subjects the predicted value plus 1.5 SDs could be used.