The clinical and immunological data of 5 patients with transient h hypogammaglobulinemia of infancy (THI) are presented and compared to published data. In THI, there is distinction between patients who can mount a specific immune response, by producing specific antibodies to bovine serum albumin, diphtheria toxin, and isohemagglutinins, and patients with more severe forms of immunodeficiency. With only 5 reported cases of THI in 11 years and analysis of more than 8000 sera-, our data support the notion that THI is a relatively rare disorder. THI can only be diagnosed with certainty in retrospect, although the ability to mount an active, specific immune response is a strong indicator for the diagnosis. We propose a more accurate definition of THI to help avoid clinical misjudgement.