Summary:Purpose: Infantile spasms (ISs) are age-dependant epileptic seizures, which may be flexor, extensor, lightning or nods, or mixed. The aim of this study was the analysis of genetic factors within the human leukocyte antigen (HLA) complex associated with ISs.Methods: Sixty-five patients diagnosed according to the established international criteria were compared with 229 healthy individuals; all of them were Mexican Mestizos. Five families were also analyzed (seven affected and five healthy sibs); HLA class I and class II antigens were typed using the standard microlymphocytotoxicity methods.Results: The findings showed female gender preference (2:1). Two thirds were symptomatic, and prevalent seizures were of mixed type (67%). A strong association with HLA-DR17 was detected in the IS group (pc < 0.01; OR ‫ס‬ 3.6; EF ‫ס‬ 0.20). DR17 was also found increased in the symptomatic patients (p ‫ס‬ 0.009; OR ‫ס‬ 3.16) and in those with other types of seizures (p ‫ס‬ 0.001; OR ‫ס‬ 2.0). Conversely, HLA-DQ6 was significantly decreased (pc < 0.002; PF ‫ס‬ 0.37) in the total and in the symptomatic groups (p < 0.01). Haplotype linkage was not confirmed in the families; however, those with more than one affected sib shared at least one haplotype.Conclusions: These findings suggest the contribution of DR locus to the susceptibility and the participation of DQ region in the resistance to IS. Severity seems also to be influenced by HLA-DR17, and therefore class II typing may be a helpful tool for disease prognosis.