BACKGROUND: Common variable immunodeficiency (CVID) is defined by hypogammaglobulinemia and increased susceptibility to infections. The gene defect responsible for CVID remains unknown. METHODS: During the course of their CVID disease, a female and three male patients developed microcytic anemia. The investigation of this anemia forms the basis for this report. RESULTS: Reticulocyte globin chain synthesis studies revealed the abnormal alpha/beta ratios that are pathognomonic of thalassemia. Through transcriptional analysis of the glucose-6-phosphate-dehydrogenase (G6PD) locus of the active X-chromosome in blood cells, we determined that the female patient has clonal reticulocytes, platelets, granulocytes, and B and T lymphocytes. CONCLUSIONS: The simultaneous presence of globin synthesis abnormalities and panhypogammaglobulinemia suggests that a common insult at the stem cell level could contribute to the development of CVID and acquired thalassemia.