To determine sex-related differences in the severity of host inflammatory reaction to cysticercosis, we studied computed tomographic findings in 100 patients with parenchymal neurocysticercosis and cerebrospinal fluid results in 239 patients with subarachnoid neurocysticercosis. Computed tomographic and cerebrospinal fluid data in male subjects were compared with those obtained in female subjects. We found that when cysticerci are found in brain parenchyma, women develop a greater degree of inflammation; such differences disappear when cysticerci are found in the subarachnoid space. Our results point out the possibility of a factor located within brain parenchyma that accounts for the observed sex-related differences in the severity of immune response to the parasite; this factor could also play a role in the pathogenesis of other immunologically mediated diseases of the brain that may occur more frequently in women. To our knowledge, this study is the first in demonstrating that sex is a risk factor for the severity of inflammatory response within brain parenchyma to a parasitic disease.