An objective measure (Lesion Signal-to-Noise Ratio) quantifying the detectability of lesions in echographic images was employed. This measure was used to determine the performance of digital speckle reduction filters, which were applied to computer simulated ultrasound B-mode images. One linear (mean filter) and two nonlinear filters (median and L2-mean filters) have been investigated. A comparison was made between fixed and adaptive versions of these filters. The influence of the size of the filter window on the Lesion Signal-to-Noise Ratio was systematically investigated. Also, the effect of the shape of the filter window is illustrated. The difference in performance of the linear and nonlinear filters was found to be small. Adaptive filters did not perform significantly better than fixed filters. The maximum improvement of lesion detectability was in the order of 40 percent. The choice of a correct window size was critical. For all types of filters, an optimum window size appeared to be present in the curves relating the Lesion Signal-to-Noise Ratio to this size.