The central problem of complex inheritance is to map oligogenes for disease susceptibility, integrating linkage and association over samples that differ in several ways. Combination of evidence over multiple samples with 1,037 families supports loci contributing to asthma susceptibility in the cytokine region on 5q [maximum logarithm of odds (lod) ؍ 2.61 near IL-4], but no evidence for atopy. The principal problems with retrospective collaboration on linkage appear to have been solved, providing far more information than a single study. A multipoint lod table evaluated at commonly agreed reference loci is required for both collaboration and metaanalysis, but variations in ascertainment, pedigree structure, phenotype definition, and marker selection are tolerated. These methods are invariant with statistical methods that increase the power of lods and are applicable to all diseases, motivating collaboration rather than competition. In contrast to linkage, positional cloning by allelic association has yet to be extended to multiple samples, a prerequisite for efficient combination with linkage and the greatest current challenge to genetic epidemiology. T he central problem of complex inheritance is to map oligogenes for disease susceptibility. Even the largest study has low power to map genes of small effect by the complementary methods of linkage and allelic association, and a confirmatory sample is necessary even for genes of large effect. Often the significance of multiple samples is controversial. These considerations favor combination of evidence, for which there are three approaches called meta-analysis, prospective collaboration, and retrospective collaboration.Meta-analysis typically uses published summaries and so is comprehensive. It would be the method of choice if publication did not favor positive results and the data were summarized in a way that distinguishes information, effect, and location, like lod scores for major loci. Unfortunately, studies of complex inheritance differ widely in phenotype definition, selection of families or cases and controls, choice of marker loci, and statistical analysis. The only common denominator is a nominal significance level, which is usually not specified unless it lies beyond an arbitrary threshold. Such material, although peer-reviewed, is subject to many errors and biases but can be used to define candidate regions (1).Prospective collaboration is typified by national consortia like the Collaborative Study on the Genetics of Asthma (2). Each member of the consortium tries to follow the same protocol, including phenotype definitions, sampling scheme, and markers. This effort assures uniformity but makes no use of the many studies with different but equally defensible protocols.Retrospective collaboration is typified by the international Consortium on Asthma Genetics (COAG). Each member of the consortium provides data published independently with no agreement about protocol. The analysis must be coherent, or else would be no more than a meta-analysis of a subse...