There is a powerful evidence suggesting that etiology and pathogenesis of systemic lupus erythematosus has both genetic and environmental components. Unfortunately, understanding the genetics of lupus has been impeded by knowing the pattern of inheritance. Indeed, a complex mode of inheritance for the lupus disease phenotype is suggested by the known characteristics of this disorder. Twenty-five multiplex pedigrees for lupus have been enrolled and have been used to evaluate power to reveal linkage. The power to find linkage in these pedigrees is greater for autosomal recessive than for autosomal dominant modes of inheritance. Once 100 similar pedigrees are available for analysis our results predict that linkage is likely to be present for genetic models with relatively relaxed requirements. At loci operating by autosomal recessive inheritance linkage should be detectable despite genetic homogeneity as low as 40% and penetrance as low as 50%. For loci operating by autosomal dominant inheritance genetic homogeneity must be 60% or more when penetrance is as low as 50% to be able to establish linkage. Available preliminary data are also consistent with a possible genetic linkage of Fc gamma RIIIPMN with lupus in American Black pedigrees multiplex for lupus.