Transient hypogammaglobulinemia (THI) of infancy is a common primary immunodeficiency usually resolves by 3 years of age. In this study, clinical, immunological data and outcome of 101 retrospectively diagnosed THI patients were evaluated. Majority of them suffered from recurrent respiratory infections (70.3%). Initial IgG, IgM and IgA levels were 446.7±121.5, 67.5±32.8, and 25.6±16.8 mg/dl, respectively. Patients who had lower IgG levels on admission reached normal IgG levels earlier than others. Infants who were retarded to reach age-related normal levels for IgM and IgA were found to have higher CD3+CD8+ T cells on admission. During immunoglobulin abnormalities, mean lymphocyte subset percentages and absolute counts were normal. Mean percentage of CD19+CD27+ memory B cells was 3.4±1.4% which is not significantly different from healthy children. Most of the children had protective antibody responses to tetanus (87%) and Haemophilus influenzae type B (85.7%) vaccines. Patients with low anti-tetanus responses had higher initial natural killer (NK) cell percentages probably due to recurrent viral infections or relative dominance of innate responses. Follow-up of patients with initially high NK were found to have longer duration of deficiency hence these patients' recoveries were delayed. During follow-up, 91/101 (90.1%) children produced normali levels of IgG at the end of 29.2 ± 15.2 months. The results of this study indicate that some children will achieve normal levels of IgG within 30 months of age, and some will remain IgG subclass or IgA deficient. Determination of increased NK percentages in patients with non-protective vaccine response and normal percentages of memory B cells are noteworthy novel findings.