Porcine epidemic diarrhea virus (PEDV) is the major causative agent of fatal diarrhea in piglets. To study the pathogenic features of PEDV using a mouse model, PEDV with virulence in mice is required. In pursuit of this, we adapted a tissue-culture-passed PEDV MK strain to suckling mouse brains. PEDV obtained after ten passages through the brains (MK-p10) had increased virulence for mice, and its fusion activity in cultured cells exceeded that of the original strain. However, the replication kinetics of MK and MK-p10 did not differ from each other in the brain and in cultured cells. The spike (S) protein of MK-p10 had four amino acid substitutions relative to the original strain. One of these (an H-to-R substitution at residue 1,381) was first detected in PEDV isolated after eight passages, and both this virus (MK-p8) and MK-p10 showed enhanced syncytium formation relative to the original MK strain and viruses isolated after two, four, and six passages, suggesting the possibility that the H-to-R mutation was responsible for this activity. This mutation could be also involved in the increased virulence of PEDV observed for MK-p10.