Genetic data are useful for estimating the genealogical relationship or relatedness between individuals of unknown ancestry. We present a computer program, ml-relate that calculates maximum likelihood estimates of relatedness and relationship. ml-relate is designed for microsatellite data and can accommodate null alleles. It uses simulation to determine which relationships are consistent with genotype data and to compare putative relationships with alternatives. ml-relate runs on the Microsoft Windows operating system and is available from http://www.montana.edu/kalinowski.