The effects of uncertain input data on the performance evaluation of a distribution system are analyzed. A criterion is introduced for assessing the grade of uncertainty of the results obtained in the calculation of maximum loads, voltage drops, energy losses, and characteristic reliability indices of a network if some input parameters are only guesses based on limited experience, measurements, and/or statistical data. Reasonable outputs bounds are determined based upon the shape of the function measuring the uncertainty. High uncertainty of a result obtained indicates that a re-examination of relevant uncertain input data would be recommendable for a more precise quantification. The method proposed is applied to a real life example for illustration.