A new test of inheritance, S, is proposed, which uses information from affected as well as unaffected siblings in the fam­ ily. The siblings are analyzed in terms of similarities of haplotypes. The distribution of the proposed S­test is derived under the null hypothesis of random inheritance. Mean and variance are obtained for the distribution. The test is then applied to data sets published in the literature. The results suggest some sort of linkage between haplotypes and disease genes.