This paper describes some general characteristics of the uncertainties in various fast-reactor parameters as a function of the corresponding uncertainties in nuclear data on the basis of statistical theory. By using the variational formalism, it is possible to relate the mean-square deviation of any reactivity coefficient (or reaction-rate ratio) to the covariance matrix of cross sections. The qualitative behavior of the uncertainties in fast-reactor parameters can be best illustrated geometrically in terms of the region of tolerance. The latter is the volume bound by a “hyper-ellipsoid” surface specified by the designer’s tolerance limit, the coefficients characteristic of the type of parameter in the system under consideration, and the correlation matrix. Exploratory studies and graphical illustrations are given for some cases of practical interest.