Haseman & Elston (1972) developed a simple and robust method for detecting quantitative trait loci (QTL) by their linkage to a single marker using information from sib-pairs. The method involves the regression of the squared difference between the phenotypic scores onto the proportion of alleles at the marker which are identical by descent. The availability of genetic maps of marker loci makes it possible to extend this method to incorporate information from several marker loci. Here we show that by considering identity by descent from the two parents separately, a simple method can be obtained to use information from multiple markers to estimate the proportion of alleles identical by descent for a QTL in any given location. Considered this way, the method can also be very simply extended to allow for differences in recombination frequencies between the sexes and more complicated relationships between individuals. We show by simulation that this method is as powerful as alternative least squares approaches using multiple markers (Fulker et al., 1995)  and provides more accurate estimates. It is also much easier to implement.