The aim of the present study was to investigate the etiologic factors and immunological findings in the patients with infantile spasm (IS). M Ma at te er ri ia al l a an nd d M Me et th ho od ds s: : Twenty-five infants (17 boys, 8 girls) with IS were included in the study. The clinical characteristics and underlying etiologic factors were analyzed. Neurometabolic and immunologic tests, neuroimaging and electroencephalography were performed. Twenty healthy infants were chosen as the control group. The groups were compared regarding serum immunoglobulin levels, complement proteins and T lymphocyte subgroups. R Re es su ul lt ts s: : The male/female ratio was 2/1 and mean age was 8.6±3.5 months in patients with IS. Most (88%) of the patients with IS were classified as structural/metabolic, the most frequent etiological factor being hypoxic-ischemic encephalopathy. Cerebral infarct, tuberous sclerosis, cortical dysplasia and metabolic diseases were other causative factors. All patients received adrenocorticotrophic hormone as first line treatment except two patients with tuberous sclerosis treated with vigabatrin. The patients with IS had lower median IgA, IgM, IgG, C3 and C4 levels compared to healthy control group. The distribution of lymphocyte subgroups CD3+,CD4+,CD8+ and CD19+ were not different between two groups. C Co on nc cl lu us si io on n: : Humoral and cell-mediated components of the immune system can be affected in IS. This may be due to the underlying brain disorder or to seizures, and may suggest inflammatory mechanisms taking part in IS. Immune dysregulation should be taken into consideration when treating IS.