Many prenatal and perinatal factors are hypothesized to play a role in the cause of cerebral palsy (CP). Epidemiological data implicate maternal–fetal infection and associated increase in circulating cytokines. Murine model data suggest that excitotoxic damage can produce pathological change in brain tissue consistent with lesions observed in CP. Specifically, on day 5 after birth, mouse pups injected with ibotenate, a glutamatergic analogue, develop transcortical necrosis and white matter cysts mimicking some human perinatal lesions associated with CP. The present study builds on this murine model to assess the modulating role of several cytokines on the development of excitotoxic lesions. Pups pretreated with interleukin (IL)‐1β, IL‐6, IL‐9, or tumor necrosis factor‐α developed significantly larger ibotenate‐induced cortical and white matter damage than controls; IL‐4 did not produce such an effect. In a similar manner, IL‐9–overexpressing transgenic pups developed ibotenate‐induced brain lesions, which were significantly larger than those induced in nontransgenic control pups. Pretreatment with proinflammatory cytokines significantly increased neopallial microglial density without affecting astrocytic density; IL‐9 or IL‐4 did not produce a similar effect. To our knowledge, this is the first in vivo study to demonstrate that systemically administered proinflammatory cytokines and IL‐9 exacerbate brain lesions that are similar to those found in human infants with CP. Ann Neurol 2000; 47:54–63