Microfibrillar collagen hemostat (MCH), also known by its trade name Avitene, is commonly used to control hemorrhage during neurosurgery. Among the documented complications associated with this agent, a granulomatous foreign body reaction is rare, having been described in the central nervous system in only one previous clinical report. In the present study, the authors report the case of a 3-year-old boy who presented with a lesion which appeared to be the recurrence of a tumor 2 months after he had undergone gross-total resection for a medulloblastoma. The patient underwent resection of the presumed recurrent tumor, but histopathological analysis of the specimen revealed a granulomatous foreign body reaction to MCH and no tumor recurrence. In addition to describing the case, the authors review the surgical literature on foreign body reactions to MCH.