The accuracy of the Empirical Transfer Function Estimator (ETFE) with stochastic input has been described with a deterministic and a stochastic approach. The results for bias and variance are different, but the mean square error is infinite in both approaches. In the deterministic approach where the input is considered as a single realization of a stochastic process, a number of frequencies has no finite ETFE estimate. The average number of problematic frequencies will be constant if the number of observations increases. (C) 2003 Elsevier Ltd. All rights reserved.