A program named `MRTTOL' has been devised to help in evaluating the performance of forward looking infrared (FLIR) systems in the presence of tolerances. It is essentially a shell program to execute the NVESD analysis program `FLIR92' multiple times with randomly varied input parameters, then calculate statistics on the minimum resolvable temperature difference (MRTD) predictions of FLIR92. The input to FLIR92 is a template-style data file in which the data must be in a certain sequence within each of a set of data blocks. This lends itself well to setting up a file of tolerances with a one-to-one correspondence with each of the datum inputs. The MRTTOL program expects just this correspondence between a `nominal' data file and a `tolerance' file giving the expected variations on each of the datum in the `nominal' file. The program interprets each tolerance as being the 2-sigma variation of the associated datum in the nominal file. It repeatedly sets up a perturbed data set by randomly varying each input value, runs FLIR92 with the perturbed data set, and accumulates the horizontal, vertical, and 2-D MRTD values in an output file. After running whatever number of perturbed data sets the user calls for, the program calculates the mean and standard deviation of the MRTD at each of 20 equally spaced spatial frequencies (increment set by the user). The output lists the expected values of MRTD for cumulative probabilities at several percentages from 50% to 3-sigma.