Common variable immunodeficiency (CVID) is an antibody deficiency syndrome that often co-occurs in families with selective IgA deficiency (IgAD). Vorechovský et al. (Am J Hum Genet 64:1096-1109, 1999; J Immunol 164:4408-4416, 2000) ascertained and genotyped 101 multiplex IgAD families and used them to identify and fine map the IGAD1 locus on chromosome 6p. We analyzed the original genotype data in a subset of families with at least one case of CVID and present evidence of a CVID locus on chromosome 16q with autosomal dominant inheritance. The peak (model-based) LOD score for the best marker D16S518 is 2.83 at theta=0.07, and a 4-marker LOD score under heterogeneity peaks at 3.00 with alpha=0.68. The (model-free) NPL score using the same markers peaks at the same location with a value of 3.38 (P=0.0001).