A case report of a 16 year old boy in whom selective IgA deficiency progressed to typical common variable immunodeficiency (CVID) is described. This boy with a history of frequent but not severe respiratory tract infections was referred to hospital because of severe pleuropneumonia and decreased levels of IgA (0.23 g/L), but normal IgG and IgM levels. Lymphocyte subpopulation determination revealed a decreased proportion of CD4+ lymphocytes (30%) and an increased proportion of CD8+ lymphocytes (32%), while CD3+, CD19+ and CD16+/56+ subpopulations were normal. During the subsequent 17 months a gradual decrease in IgG (ultimate level 2.23 g/L), IgA (< 0.05 g/L) and IgM (< 0.05 g/L) levels was observed, the decrease in IgM being the slowest reflecting a constant heavy chain gene order on chromosome 14. The observation supports the thesis of a close relation of selective IgA deficiency and common variable immunodeficiency.