A simple procedure designed specifically to detect linkage for rare recessive diseases is described. The method uses information on identity by descent scores for a pair of sibs at a marker locus conditioned on the number of affected sibs in the pair. A procedure for estimating the recombination fraction is described, and a table facilitating the likelihood ratio test of linkage is provided. The method, when applied to a collection of multiplex families segregating for juvenile diabetes mellitus, suggests the possibility that this disease is linked to the HLA complex. The method is found to compare favorably to the maximum likelihood approach, for which the computer program LIPED gives a maximum lod score of 2.48 at a male and female recombination fraction of theta = 0.20.