Cerebral palsy (CP) is the most frequent neurological disorder associated with perinatal injury of the developing brain. Major brain lesions associated with CP are white matter damage (WMD) in preterm infants and cortico-subcortical lesions in term newborns. Cell therapy is considered promising for the repair of brain damage. Human umbilical cord blood mononuclear cells (hUCB-MNCs) are a rich source of various stem cells that could be of interest in repairing perinatal brain damage. Our goal was to investigate the potential of hUCB-MNCs to prevent or repair brain lesions in an animal model of excitotoxic brain injury. We induced neonatal brain lesions using intracranial injections of ibotenate, a glutamate agonist, in 5-day-old rat pups. hUCB-MNCs were injected either intraperitoneally (i.p.) or intravenously (i.v.) soon or 24 h after ibotenate injection, and their neurological effects were assessed using histology and immunohistochemistry. hUCB-MNCs injected i.p. did not reach the systemic circulation but high amounts induced a significant systemic inflammatory response and increased the WMD induced by the excitotoxic insult. This effect was associated with a significant 40% increase in microglial activation around the white matter lesion. hUCB-MNCs injected i.v. soon or 24 h after the excitotoxic insult did not affect lesion size, microglial activation, astroglial cell density, or cell proliferation within the developing white matter or cortical plate at any concentration used. We demonstrated that hUCB-MNCs could not integrate into the developing brain or promote subsequent repair in most conditions tested. We found that the intraperitoneal injection of high amounts of hUCB-MNCs aggravated WMD and was associated with systemic inflammation.