This paper investigates: 1) the sensitivity of reliability-growth models to errors in the estimate of the operational profile (OP), and 2) the relation between this sensitivity and the testing accuracy. The investigation is based on the results of a case study in which several reliability-growth models are applied during the testing phase of a software system. The faults contained in the system are known in advance; this allows measurement of the software reliability-growth and comparison with the estimates provided by the models, Measurement & comparison are repeated for various OP, thus giving information about the effect of a possible error in the estimate of the OP. The results show that: 1) the predictive accuracy of the models is not heavily affected by errors in the estimate of the OP, and 2) this relation depends on the accuracy with which the software system has been tested.454531540