Screening of close relatives of Swedish patients with selective immunoglobulin A deficiency (IgAD) and common variable immunodeficiency (CVID) for serum immunoglobulin levels has identified the positive family history of IgAD/CVID as the most significant risk factor for developing the disease. The relative risk for siblings of patients with IgAD was estimated to be approximately 50. In 12 of 34 Swedish multiplex families identified in the study, both IgAD and CVID occurred, usually CVID in the parental generation and IgAD in the subsequent generation. This proportion was much higher than expected by chance and strongly suggests that the two clinically discernible disorders represent an allelic condition, reflecting a variable expressivity of a common defect. In 27 multiplex families the disorders segregated as an autosomal dominant trait, affecting at least two generations. A high relative risk for siblings, permanent phenotype, low number of phenocopies, and common population prevalence, which makes it possible to obtain a sufficient sample size, make these immunoglobin deficiencies amenable to genetic linkage analysis. In a pilot multicenter linkage study involving 16 multiplex families with dominant transmission of IgAD/CVID, we have attempted to confirm previously reported genetic linkage of the disease susceptibility to the major histocompatibility complex (MHC) region. Using both parametric and nonparametric linkage analyses with a set of microsatellite markers at and flanking the MHC region, no evidence for linkage was found. In accordance with these results, no evidence for linkage to the MHC region was obtained by analyzing previously published segregation data at the MHC region in multiplex families with IgAD/CVID in more than one generation.(ABSTRACT TRUNCATED AT 250 WORDS)