Selective IgM deficiency (SIgMD) and IgA MGUS in a young woman are two rare disorders. IgA MGUS has not been described in patients with SIgMD. We present the first comprehensive analysis of various subsets of CD4+ T, CD8+ T cells, and B cells in a young woman with SIgMD and IgAλ MGUS. Analysis of B cell subsets revealed in creased proportions of transitional B cells, germinal center (GC) B cells, B regulatory cells (Breg), and plasmablasts (PB), and decreased proportions of marginal zone (MZ) B cells. BAFF-R expression on both naïve and memory B cells was increased. CD4+ and CD8+ effector memory cells were decreased, whereas CD4+ and CD8+ naïve T cells were increased. These abnormalities in B cell subsets and plasmablasts are not observed in SIgMD, therefore appears be influenced by MGUS. No correlation was observed with changes in the levels of monoclonal IgA and serum IgM levels over nine years follow-up suggesting that SIgMD is likely to be primary rather than secondary to MGUS. These observations also suggest that IgAλ MGUS and perhaps other MGUS may occur at a young age in association with selective IgM deficiency. The abnormalities in B cell subsets may have a predictive value for progression to multiple myeloma.