Lightning location systems (LLSs) are often used to obtain parameters of lightning flashes. However, the imperfect detection efficiency (DE) of these systems affects the accuracy of the results. In this paper, we focus on the effects of the imperfect DE on the probability distribution of interstroke intervals. Two DE models are considered here: one dependent on the peak amplitude of return stroke currents and the other independent of any stroke parameter. For these two models, the results show that the distribution of interstroke intervals obtained from strokes detected by an LLS depends essentially on the average DE in %. In addition, we demonstrate that the average DE has a noticeable influence on the geometric mean (GM) of the measured interstroke intervals. Finally, we provide a mathematical expression for estimating the DE of an LLS; this expression uses the GM of the interstroke intervals obtained from the LLS data and assumes that the distribution of interstroke intervals for the geographical location of concern is known.