Tansient hypogammaglobulinemia of infancy (THI) is characterized by an abnormal delay in the onset of immunoglobulin synthesis. In the present study, clinical and immunological data and outcome of 30 patients with THI are presented. Between 1995-2001, 464 patients with frequent upper and lower airway infections admitted to the Pediatric Immunology and Allergy outpatient clinic; 30 of them (aged between 6-46 months) were diagnosed as THI. Patients had periodic evaluations at 3-6 month intervals until their condition had improved clinically and immunoglobulins had normalized. In 21 patients immunoglobulin levels reached normal age-matched levels at a median age of 27 months. In nine patients (aged 25-57 months) immunoglobulin levels were still under 2SD for age and various IgG subclass deficiencies were detected in five of them. The results of the present study indicated that THI is not a rare disease. Long-term follow-up with detailed clinical and laboratory evaluations is needed in these patients.