The search for genes underlying complex traits has been difficult and often disappointing. The main reason for these difficulties is that several genes, each with rather small effect, might be interacting to produce the trait. Therefore, we must search the whole genome for a good chance to find these genes. Doing this with tens of thousands of SNP markers, however, greatly increases the overall probability of false-positive results, and current methods limiting such error probabilities to acceptable levels tend to reduce the power of detecting weak genes. Investigating large numbers of SNPs inevitably introduces errors (e.g., in genotyping), which will distort analysis results. Here we propose a simple strategy that circumvents many of these problems. We develop a set-association method to blend relevant sources of information such as allelic association and Hardy-Weinberg disequilibrium. Information is combined over multiple markers and genes in the genome, quality control is improved by trimming, and an appropriate testing strategy limits the overall false-positive rate. In contrast to other available methods, our method to detect association to sets of SNP markers in different genes in a real data application has shown remarkable success.The current emphasis on searching for disease susceptibility genes is carried out by association to tens of thousands of SNP markers (Collins et al. 1998). Such association analyses may be carried out in a variety of data designs, for example, by testing for differences in SNP allele frequencies between affected and unaffected individuals (case-control studies), or by comparing whether a SNP allele is transmitted to an affected offspring more or less often than expected by chance (the transmission disequilibrium test, TDT; Spielman and Ewens 1996). Because complex traits presumably arise from multiple interacting genes located throughout the genome, it would be appropriate to search for sets of marker loci in different genes and to analyze these markers jointly rather than testing each marker in isolation. Forming haplotypes over multiple neighboring markers in one gene can increase the power of gene mapping studies (Fallin et al. 2001), as can scan statistics ; but these methods only work locally in a given genomic region.Most current approaches essentially evaluate one SNP marker at a time, that is, by focusing on its marginal effect on disease. Those SNPs with a significant association to disease are taken to be close to or within susceptibility genes. Testing each SNP for association with disease leads to a locus-specific probability of a false-positive result (type I error). Such a type I error can easily be inflated when large numbers of SNPs are tested simultaneously and treated independently (Risch and Merikangas 1996); the problems involving such multiple testing and its effect on the genomewide type I error are the subject of a presently ongoing debate (Lin et al. 2001). For genomewide linkage analysis, appropriate measures have been developed to keep this problem u...